gerrit: some fixes for the rfc script

gerrit: some fixes for the rfc script

In rebase_changes():

- Using "--all" for git fetch is bogus: the scope of this quantification is not
  over the tracked refs of the upsteam repo (which we do want, but that kind
  of universal quantification is already implied by the notion of "tracking"),
  but over the set of all the registered remotes, which is an annoying side effect.

- $EDITOR is weaker than the git configuration settings. For the purpose of the
  script, we need to override any kind of setting, and that can be done
  via $GIT_EDITOR.

Cf. git-fetch(1), git-commit(1).

Change-Id: Ia91ee5c4d42dce9248f26f7697528e3c97355871
BUG: 3234
Reviewed-on: http://review.gluster.com/86
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Anand Avati <avati@gluster.com>