Getting the current branch name fails in macOS

[Mayeu]

Hello,

I am using macOS 10.11.6, and I had trouble to use TopGit. The most basic tg info ended up into a not a TopGit-controlled branch when the branch was created via tg create.

After taking a pick in the source code, I end up finding the cause. The way the current branch name is inferred does not work with macOS'sed:

- ansible-consul git:(t/readme)
🍵  $ git symbolic-ref HEAD 2>/dev/null | sed 's#^refs/\(heads\|top-bases\)/##'
refs/heads/t/readme

I tried to fix the regex but without success, so I ended up installing gnu-sed via homebrew:

$ brew install gnu-sed --with-default-names

And now everything is working.

I believe it would be worth to make the regex also work on macOS, or making it explicit somewhere than GNU-sed is needed.

Now I am getting back to trying and playing with TopGit :) Thank you for your works!