builtin/refs: add "create" subcommand

builtin/refs: add "create" subcommand

The "update" subcommand cannot only update an existing reference, but it
can also create new branches and delete existing branches by specifying
the all-zeroes object ID as either old or new value. Despite that, we
already have the "delete" subcommand as a handy shortcut so that a user
can easily delete a branch. This relieves them of needing to understand
the more arcane uses of the "update" command, and of counting the number
of zeroes they need to pass.

But while we have a "delete" subcommand, we don't have an equivalent
that would allow the user to create a new branch, which creates a
certain asymmetry.

Add a new "create" subcommand to plug this gap.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>