git-gui: Display database stats (count-objects -v) on demand.

git-gui: Display database stats (count-objects -v) on demand.

Its nice to know how many loose objects and roughly how much disk space
they are taking up, so that you can guestimate about when might be a
good time to run 'Compress Database'.  The same is true of packfiles,
especially once the automatic keep-pack code in git-fetch starts to
be more widely used.

We now offer the output of count-objects -v in a nice little dialog
hung off the Repository menu.  Our labels are slightly more verbose
than those of `count-objects -v`, so users will hopefully be able
to make better sense of what we are showing them here.

We probably should also offer pack file size information, and data
about *.idx files which exist which lack corresponding *.pack files
(a situation caused by the HTTP fetch client).  But in the latter
case we should only offer the data once we have way to let the user
clean up old and inactive index files.

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