git-gui: Use progress meter in the status bar during index updates

git-gui: Use progress meter in the status bar during index updates

If we are updating the index to stage or unstage changes or reverting
files in the working directory we can use the progress handling parts
of our status bar to perform this display work, reducing the amount of
code duplication we have in the index handling module.

Unfortunately the status bar is still a strict approximation as it is
unable to know when git-update-index has processed the data we fed to
it.  The progress bar is actually a progress of the pipe buffer filling
up in the OS, not of the actual work done.  Still, it tells the user we
are working and that has some value.

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