git-gui: Make sure we get errors from git-update-index

git-gui: Make sure we get errors from git-update-index

I'm seeing a lot of silent failures from git-update-index on
Windows and this is leaving the index.lock file intact, which
means users are later unable to perform additional operations.

When the index is locked behind our back and we are unable to
use it we may need to allow the user to delete the index lock
and try again.  However our UI state is probably not currect
as we have assumed that some changes were applied but none of
them actually did.  A rescan is the easiest (in code anyway)
solution to correct our UI to show what the index really has
(or doesn't have).

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