Make GitUp compatible with worktrees

[szhu]

Hey there, I appreciate this fork of GitUp! If it's possible, can you look into upgrading libgit2 so that GitUp is compatible with Git worktrees?

I would love to use worktrees in my workflow and the lack of GitUp support is the only reason I'm not using them yet. (I currently just clone the same repo multiple times -- not super efficient.)

See git-up#57 for a discussion of this. It looks like one of the commenters was able to just do a version bump to make worktrees work out of the box.

Thanks in advance!