git-p4: update multiple shelved change lists

git-p4: update multiple shelved change lists

--update-shelve can now be specified multiple times on the
command-line, to update multiple shelved changelists in a single
submit.

This then means that a git patch series can be mirrored to a
sequence of shelved changelists, and (relatively easily) kept in
sync as changes are made in git.

Note that Perforce does not really support overlapping shelved
changelists where one change touches the files modified by
another. Trying to do this will result in merge conflicts.

Signed-off-by: Luke Diamand <luke@diamand.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>