git-gui: Fix detaching current branch during checkout

git-gui: Fix detaching current branch during checkout

If the user tried to detach their HEAD while keeping the working
directory on the same commit we actually did not completely do
a detach operation internally.  The problem was caused by git-gui
not forcing the HEAD symbolic ref to be updated to a SHA-1 hash
when we were not switching revisions.  Now we update the HEAD ref
if we aren't currently detached or the hashes don't match.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>