git-gui: Run the pre-commit hook in the background.

git-gui: Run the pre-commit hook in the background.

I started to notice on Windows that commits took a lot longer to get
going than on my Mac OS X system.  The real reason is the repositories
that I'm testing with on Windows all enabled the standard pre-commit hook
while my test repository on Mac OS X doesn't have it executable (so its
not running).  So the Windows repositories are spending this
lag time running that hook.

Now we run the pre-commit hook in the background, allowing the UI to
update and tell the user we are busy doing things.

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