Drop diffutils dependency

[NathanReb]

We are currently using the diffutils project to print API diffs as human readable text diffs. diffutils is not released to opam and it prevents us from releasing api-watch.

Vendoring the whole diffutils project would be a bit overkill as we're only using a very small part of it.

We would like to instead embed the types and functions we use directly in the project.

This makes sense as it is likely that our need for custom diff printing utilities will grow as we move toward a proper structural diff representation of API diffs.

What to import

All of our uses of Diffutils are contained within our Text_diff module. All the types we use come from Diffutils.Diff:

• the type t that's used to represent a textual diff
• the printer type used to represent the different pretty printers
• the printer function that creates a value of the above type
• the git_printer value that prints textual diff similarly to git diff
• the pp function that takes a printer and returns a function to pretty print textual diffs

For now we'll simply re-implement those exactly like they are defined in Diffutils, we'll make adjustment to suit our needs in the future.

Those parts of the code are already tested so we won't need to write new ones there. Making sure our existing test pass should be enough.

[azzsal]

Hello,

I am an outreachy applicant, I am interested in contributing to this project. I have just built the project successfully as requested.

Could this issue be assigned to me, please ?

Thank you!

[panglesd]

Hello @azzsal !

Yes, let this issue be assigned to you!