git-gui: Start UI with the index locked.

git-gui: Start UI with the index locked.

Because we immediately start a rescan operation, but do so slightly
delayed (by 1 ms, to let the UI show before we start forking off
git processes), we can't let the user try to activate any of the
restricted GUI commands before the 1 ms timer expires and we kick
off the rescan.

So now we lock the index before we enter the Tk event loop, ensuring
that it is impossible for the user to inject a conflicting UI event
before our rescan can begin.

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