git-gui: Only populate a fetch or push if we have an action.

git-gui: Only populate a fetch or push if we have an action.

Don't offer to fetch from a remote unless we have at least one Pull:
line in its .git/remotes/<name> file or at least one configuration
value for remote.<name>.fetch.  Ditto for push.

Users shouldn't be fetching or pushing branch groups unless they
have them configured; anything else is just crazy.

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