git-gui: Provide an after-rescan script to rescan.

git-gui: Provide an after-rescan script to rescan.

There are some situations where we need to run rescan and have it do
more than just updating the status in the UI when its complete.  To
help with that this changes the rescan procedure to take a script which
it will run at the global level as soon as the rescan is done and the
UI has finished updating with the results.  This is useful for example
if we performed a rescan as part of a commit operation; we can go back
to the commit where we left off when the rescan got initiated.

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