Add support for apply-patch

If libgit2 adds support for git's builtin/apply.c, we would be able to use libgit2 (actually I'm more interested in rugged) to apply patches received by email to a git repository.

[dannygreg]

Just adding a 👍 would love this for Mac.

[felipellrocha]

+1

[necrosis]

Agree

[aptx4869]

+1

[csware]

Cross reference #2280

[jacquesg]

Also #2296 (which I haven't gotten round to)

[GrahamTheCoder]

+1

[epixa]

+1

For others looking, it looks like #3223 is the latest attempt to get this started.

[brandonio21]

Looks like the corresponding PR was closed and this feature is now in master. To close out this task, perhaps add it to the documentation? (I cannot find git_apply__patch using the online documentation search function)

[ethomson]

#3223 only adds support for reading a patch files into the git_diff representation. There's not yet an API to take a workdir/index/etc and modify it based on that in-memory representation.

[brandonio21]

I know that applying to the worktree is not yet complete, but is there interest in making git_apply__patch itself available for consumption, @ethomson ? Thanks!

[tiennou]

Support for this has landed as part of #4705, so this is fixed. Feel free to open issues if you find missing functionality though.