Handle new descriptions of detached head introduced in git 1.8.3

see git gc8183cd builtin/branch.c

There are 4 types in total:
(no branch, rebasing %s)
(no branch, bisect started on %s)
(detached from %s)
(no branch)

Conflicts:
	GitUI/FormBrowse.cs
	GitUI/FormPull.cs