Add support for git diff

[j-martin]

Is your feature request related to a problem? Please describe.
Tools like https://github.com/jeffkaufman/icdiff are able to act like a git diff viewer. Diffsitter would be exceptionally good for that purpose.

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Can be achieve manually via diffsitter <(git show hashA:myFile) <(git show hashB:myFile)

[ClashTheBunny]

Is this resolved with #157?

[j-martin]

Yes, it works great!

[0xdevalias]

See also:

• Support git identifiers when running under git diff #627