git-p4: add unshelve command

git-p4: add unshelve command

This can be used to "unshelve" a shelved P4 commit into
a git commit.

For example:

  $ git p4 unshelve 12345

The resulting commit ends up in the branch:
   refs/remotes/p4/unshelved/12345

If that branch already exists, it is renamed - for example
the above branch would be saved as p4/unshelved/12345.1.

git-p4 checks that the shelved changelist is based on files
which are at the same Perforce revision as the origin branch
being used for the unshelve (HEAD by default). If they are not,
it will refuse to unshelve. This is to ensure that the unshelved
change does not contain other changes mixed-in.

The reference branch can be changed manually with the "--origin"
option.

The change adds a new Unshelve command class. This just runs the
existing P4Sync code tweaked to handle a shelved changelist.

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