git-gui: Suggest when running 'git gc' may be worthwhile.

git-gui: Suggest when running 'git gc' may be worthwhile.

Users often forget to repack their object database, then start to
complain about how slow it is to perform common operations after
they have collected thousands of loose objects in their objects
directory.  A simple repack usually restores performance.

During startup git-gui now asks git-count-objects how many loose
objects exist, and if this number exceeds a hardcoded threshold
we suggest that the user compress the database (aka run 'git gc')
at this time.  I've hardcoded this to 2000 objects on non-Windows
systems as there the filesystems tend to handle the ~8 objects
per directory just fine.  On Windows NTFS and FAT are just so slow
that we really start to lag when more than 200 loose objects exist,
so the hardcoded threshold is much lower there.

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