git-gui: Supply progress feedback when running update-index.

git-gui: Supply progress feedback when running update-index.

The git-update-index process can take a while to process a large
number of files; for example my laptop would probably need almost
an hour to chug through 20,000 modified files.  In these incredibly
large cases the user should be given at least some feedback to let
them know the application is still working on their behalf, even if
it won't them do anything else (as the index is locked).

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