rev-parse: --abbrev-ref option to shorten ref name

rev-parse: --abbrev-ref option to shorten ref name

This applies the shorten_unambiguous_ref function to the object name.
Default mode is controlled by core.warnAmbiguousRefs. Else it is given as
optional argument to --abbrev-ref={strict|loose}.

This should be faster than 'git for-each-ref --format="%(refname:short)" <ref>'
for single refs.

Signed-off-by: Bert Wesarg <bert.wesarg@googlemail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>