gitk: Handle detached heads better

gitk: Handle detached heads better

This draws the currently checked-out head with a yellow circle, as
suggested by Linus Torvalds, and fixes various places in the code
where we assumed that the current head always had a branch.  Now we
can display the fake commits for local changes on a detached head.

Signed-off-by: Paul Mackerras <paulus@samba.org>