git-gui: Allow the user to disable diff stat summary during pull.

git-gui: Allow the user to disable diff stat summary during pull.

Because the pull diffstat summary can take as long as the pull itself
some users may just choose to disable the summary and save themselves
an extra few seconds during each pull.  This is especially true if the
user really doesn't care about the other files being modified, as due
to their project organizational structure they aren't really responsible
for their content.

This adds an option to the options panel which lets the user disable
the diffstat summary (and thus we pass --no-summary to git-pull) but
there does appear to be a bug in the config saving code where we did
not set the local repo config differently from the global config.

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