git-gui: Fix the after callback execution in rescan.

git-gui: Fix the after callback execution in rescan.

The rescan function receives a callback command
as its parameter, which is supposed to be executed
after the scan finishes. It is generally used to
update status. However, rescan may initiate a
loading of a diff, which always calls ui_ready after
completion. If the after handler is called before
that, ui_ready will override the new status.

This commit ensures that the after callback is
properly threaded through the diff machinery.

Since it uncovered the fact that force_first_diff
actually didn't work due to an undeclared global
variable, and the desired effects appeared only
because of the race condition between the diff
system and the rescan callback, I also reimplement
this function to make it behave as originally
intended.

Signed-off-by: Alexander Gavrilov <angavrilov@gmail.com>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>