git-gui: Don't load the global options unless necessary.

git-gui: Don't load the global options unless necessary.

Since git-repo-config will supply us a union of both the global and
the local repository configuration data when we invoke it during startup
there is no reason to go get the global configuration with an extra call
to repo-config unless the user is trying to view & edit all options in
the options dialog.

Since skipping this extra repo-config invocation save us a little bit of
time its nice to be able to avoid it when we are invoked as git-citool
and won't be running very long.

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