Adds new option "--update-shelve CHANGELIST" which updates
an existing shelved changelist.

The original changelist must have been created by the current user.

This allows workflow something like:

   hack hack hack
   git commit
   git p4 submit --shelve
   $mail interested parties about shelved changelist
   make corrections
   git commit --amend
   git p4 submit --update-shelve $CHANGELIST
   $mail interested parties about shelved changelist
   etc
Signed-off-by: NLuke Diamand <luke@diamand.org>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>