Fix #133 -- Handle git diff mnemonic prefixes

[amureki]

Git diff headers may use prefixes other than a/ and b/, for example, when diff.mnemonicPrefix=true is configured:

diff --git c/file.py i/file.py

Relint now parses diff headers with arbitrary non-empty prefixes, so both --diff and --git-diff keep working with mnemonic/custom prefixes.

I added parser-level coverage and an integration regression test for --git-diff with diff.mnemonicPrefix=true.

Other references:

• Config that changes prefixes: https://git-scm.com/docs/diff-config#Documentation/diff-config.txt-diffmnemonicPrefix
• git diff options around prefixes: https://git-scm.com/docs/git-diff

[codingjoe]

Hm... is there an advantage for us to extend the parser instead of more closely defining the diff output?

[amureki]

Hm... is there an advantage for us to extend the parser instead of more closely defining the diff output?

Yeah, this is a good open question.
The reason I went with parser-side is that relint also supports --diff, where the diff comes from stdin.
In that mode, relint cannot control how git diff was invoked, so a user (like me) piping output from Git with diff.mnemonicPrefix=true / other custom prefixes would still hit the same parser issue.

We can force prefixes, but it will only fix own relint's calls. As long as we want to support git diff and all documented non-default prefixes, we'd need to deal with the parser extension...

But it is your call. :)

[codingjoe]