possible enhancement on testing and maintenance

[maxisam]

Thanks for this great application. I totally understand it is hard to keep maintain an OSS project. However, I really need certain functions from the PRs.

So here is my fork https://github.com/maxisam/git-crypt

I mainly fixed the build and add the changes to handle worktree issues and merge issues.

Also add some simple integration test to verify the changes so we can all feel better with the PR.

If @AGWA like some updates I made (ex: testing script), feel free to reply and I will make PR after.

side notes: I don't plan to make a lot of updates but only stuffs important to me because I haven't used C++ for 20ish years. 😄