show-ref: introduce --branches and deprecate --heads

show-ref: introduce --branches and deprecate --heads

We call the tips of branches "heads", but this command calls the
option to show only branches "--heads", which confuses the branches
themselves and the tips of branches.

Straighten the terminology by introducing "--branches" option that
limits the output to branches, and deprecate "--heads" option used
that way.

We do not plan to remove "--heads" or "-h" yet; we may want to do so
at Git 3.0, in which case, we may need to start advertising upcoming
removal with an extra warning when they are used.

Signed-off-by: Junio C Hamano <gitster@pobox.com>