Add a promote client command

Takes one or more changes and promotes them to the head of the queue.

Also, change the command line syntax for the enqueue command to accept
change IDs in the form 'change,patchset' in order to match the syntax
of promote, as well as be potentially more compatible with future
triggers.

Change-Id: Ic7ded9587c68217c060328bf4c3518e32fe659e3
Merge "Add a promote client command"