git-gui: Sort the list of paths being updated in the index.

git-gui: Sort the list of paths being updated in the index.

Its a little surprising to see the UI update the icons for files
in random order, due to the fact that the files are updating in
the order they appear within the array (which is based on a hash
function and not order).  So sort the list of files before we send
any to update-index so the order of operation is means something to
the user.

Signed-off-by: Shawn O. Pearce <redacted>