git-gui: Only reshow diff when really necessary.

git-gui: Only reshow diff when really necessary.

I noticed that we were reshowing the current diff during a commit;
this occurs because we feed every added and modified file through
update-index just before commit.  During the update-index process
we reshow the current diff if the current file in the diff pane
was one of those added or modified files we reprocessed.  This
just slows down the UI more than is necessary.

So refactoring update_index so that we don't call reshow_diff
from within that code; instead we do it at a higher level.

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