Change `current_branch_name` to use short branch name.

Having a branch such as `feature/full_branch_name` only reports the branch name as `full_branch_name`. This gives you the full branch name.

Honestly I didn't run the specs locally I keep getting a weird error.
~~~
blaineschmeisser@Blaines-MacBook-Pro [11:10:20] [~/sites/devup/apps/gitsh] [feature/full_branch_name]
-> % ./autogen.sh 
configure.ac:4: error: Autoconf version 2.69 or higher is required
configure.ac:4: the top level
autom4te: /usr/bin/gm4 failed with exit status: 63
aclocal: /usr/bin/autom4te failed with exit status: 63
blaineschmeisser@Blaines-MacBook-Pro [11:10:25] [~/sites/devup/apps/gitsh] [feature/full_branch_name]
-> % autoconf --version
autoconf (GNU Autoconf) 2.69
Copyright (C) 2012 Free Software Foundation, Inc.
License GPLv3+/Autoconf: GNU GPL version 3 or later
<http://gnu.org/licenses/gpl.html>, <http://gnu.org/licenses/exceptions.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.

Written by David J. MacKenzie and Akim Demaille.
~~~