git branches with / in it are cut-off

[dgollub]

When a git branch has a / in it, promptd will not display the name correctly.

Example:

dkg@dkg:~/D/o/django-grappelli [2.7.x]$ cat .git/HEAD 
ref: refs/heads/dev/2.7.x
dkg@dkg:~/D/o/django-grappelli [2.7.x]$ git branch -v
* dev/2.7.x ded3430 Merge pull request #603 from mochawich/patch-1
  master    52ba68c changelog update
dkg@dkg:~/D/o/django-grappelli [2.7.x]$ promptd-vcs 
[2.7.x]

The issue is in git.d on line 148

// If we're on a branch head, .git/HEAD will look like
// ref: refs/heads/<branch>
if (headSHA.startsWith("ref:"))
    return headSHA.baseName;

The return headSHA.baseName will cut off at the wrong /.