support for git worktree

[namtab00]

$(GitBranch) has incorrect (default?) value of master when the GitInfo targets are invoked inside a git worktree.

[namtab00]

@kzu Hi, sorry for the ping.. Are you still supporting this repo?

[kzu]

Yes, I am. But for cases like this, I'd gladly take a PR. It's not high on my list and it seems to be a sparingly used feature (git worktree I mean). I've never used it myself in any of the git repositories I work on (or contribute to).

[kzu]

There doesn't seem to be high interest in this particular enhancement. Please do reopen and we'd welcome a PR if this is still important to you.

Thanks!