Add a "tag" command so we can run commands at the changelist level,
which is extra useful for stuff involving moves. This doesn't create a
Perforce label/tag, just creates a named alias in memory for the latest
changelist at that time. Tags are cleared by the "cd" command just
like the internal edit counter.
E.g.:
add foo
edit foo
tag apple
edit foo
branch foo@apple bar
branch foo#2 baz
test equal bar baz