Add git-crypt merge driver to support secret files merging (refactored #107)

[jirutka]

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This PR is a refactoring of PR #107. The original PR mixes multiple unrelated changes in a single commit, including some maybe unwanted changes and code-style violations, which is probably the reason why it’s still not merged.

This PR contains only and only the changes needed for the merge driver support with minimum changes to the existing code (separated into an extra commit), so it should be easy for review. I’ve also fixed a few shortcomings in the original PR.

[brandsimon]

Will this fix git add -p?

[rpasche]

Any news here? I would love to see this merged but I also have to test it myself.

[Quentin-M]

Couple of years since my last comment on this feature.. Any plan to merge @AGWA? :)

For anyone lurking, @jirutka's solution works like a charm - simply compile it, place it in your PATH and git-crypt unlock again which will update your .git/config with the proper [merge "git-crypt{-<key>}"] blocks and allow for git rebase and git merge.

[joaosa]

@AGWA This would be incredibly useful, as it addresses a common hinderance to using git-crypt. Is there any chance this could get merged? :)

[YellowOnion]

Maybe someone else can take over maintaining the project, There's 21 pull requests open, and this one is basically an essential feature.