git-gui: Internalize symbolic-ref HEAD reading logic

git-gui: Internalize symbolic-ref HEAD reading logic

To improve performance on fork+exec impoverished systems (such as
Windows) we want to avoid running git-symbolic-ref on every rescan
if we can do so.  A quick way to implement such an avoidance is to
just read the HEAD ref ourselves; we'll either see it as a symref
(starts with "ref: ") or we'll see it as a detached head (40 hex
digits).  In either case we can treat that as our current branch.

Signed-off-by: Shawn O. Pearce <redacted>