Add check for equal fields in edit command

[Silvernitro]

Closes #152

Changes

• When executing EditCommand, check if the edited module is the same as the original one. If it is, display an error message instead of continuing to "edit" it
• Add test for this + tweak one existing test that's affected by this new change
• Minor: I noticed that there was a lot of repeated code in EditCommandTest to retrieve modules from the test model, so I've abstracted it out to 2 fxns

[codecov-io]

Codecov Report

Merging #175 into master will increase coverage by 0.02%. The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master     #175      +/-   ##
============================================
+ Coverage     75.40%   75.43%   +0.02%     
- Complexity      565      566       +1     
============================================
  Files            93       93              
  Lines          1720     1722       +2     
  Branches        175      176       +1     
============================================
+ Hits           1297     1299       +2     
  Misses          341      341              
  Partials         82       82              

Impacted Files	Coverage Δ	Complexity Δ
...main/java/seedu/address/commons/core/Messages.java	0.00% <ø> (ø)	0.00 <0.00> (ø)
...java/seedu/address/logic/commands/EditCommand.java	95.38% <100.00%> (+0.14%)	14.00 <0.00> (+1.00)

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