[Resolve #1498] Tweak diff output

[alex-harvey-z3q]

In order to produce output that does not confuse syntax highlighters that recognise diff-formatted output, it is decided here to replace the "-" character (which would often cause a diff highlighter to colour the line as red) instead with "~" which would tend to be ignored by diff highlighters.

Showing GitHub's rendering of before- and after- output:

Before:

(Note that lines are highlighted in red that should not be.)

*****************************************************************************************                                                                                                             
--> Difference detected for stack flink-prod!                                                                                                                                    
-----------------------------------------------------------------------------------------                                                                                                             
Config difference for flink-prod:                                                                                                                                          

--- deployed                                                                                                                                                                                          
+++ generated                                                                                                                                                                                         
@@ -7,4 +7,4 @@                                                                                                                                                                                       
   Project: Datalake                                                                                                                                                                                  
   SourceControlPath: fsa/sceptre-environment/prod/datalake-prod                                                                                                              
   Team: DevOps                                                                                                                                                                                       
-  Version: 7.0.0                                                                                                                                                                                     
+  Version: 8.0.0                                                                                                                                                                                     
-----------------------------------------------------------------------------------------                                                                                                             
No template difference                                                                                                                                                                                
*****************************************************************************************                                                                                                             
--> Difference detected for stack flink-prod!                                                                                                                                       
-----------------------------------------------------------------------------------------
Config difference for flink-prod:

After:

*****************************************************************************************                                                                                                             
==> Difference detected for stack flink-prod!                                                                                                                                    
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Config difference for flink-prod:                                                                                                                                          

--- deployed                                                                                                                                                                                          
+++ generated                                                                                                                                                                                         
@@ -7,4 +7,4 @@                                                                                                                                                                                       
   Project: Datalake                                                                                                                                                                                  
   SourceControlPath: fsa/sceptre-environment/prod/datalake-prod                                                                                                              
   Team: DevOps                                                                                                                                                                                       
-  Version: 7.0.0                                                                                                                                                                                     
+  Version: 8.0.0                                                                                                                                                                                     
*****************************************************************************************                                                                                                             
No template difference                                                                                                                                                                                
*****************************************************************************************                                                                                                             
==> Difference detected for stack flink-prod!                                                                                                                                       
*****************************************************************************************                                                                                                             
Config difference for flink-prod:

PR Checklist

• [x] Wrote a good commit message & description [see guide below].
• [x] Commit message starts with [Resolve #issue-number].
• [x] Added/Updated unit tests.
• [ ] Added/Updated integration tests (if applicable).
• [x] All unit tests (poetry run tox) are passing.
• [x] Used the same coding conventions as the rest of the project.
• [x] The new code passes pre-commit validations (poetry run pre-commit run --all-files).
• [x] The PR relates to only one subject with a clear title. and description in grammatically correct, complete sentences.

Approver/Reviewer Checklist

• [ ] Before merge squash related commits.

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