subrepo: lazier git push logic
Avoids calls to git push when the revision is already known to be
in the remote repository. Now, when using a read-only git subrepo,
git will never need to talk to its upstream repository.
$ echo '[Section]' >> $HGRCPATH
$ echo 'KeY = Case Sensitive' >> $HGRCPATH
$ echo 'key = lower case' >> $HGRCPATH
$ hg showconfig Section
Section.KeY=Case Sensitive
Section.key=lower case