git-gui: Allow update_index to also run a script when it completes.

git-gui: Allow update_index to also run a script when it completes.

Like rescan we also have cases where we need to perform a script
after we have finished updating a number of files in the index.  By
changing the parameter structure of update_index we can easily pass
through any script we need to run afterwards, such as picking up
in the middle of a commit, or finishing what is left of a rescan.

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