describe --always: fall back to showing an abbreviated object name

describe --always: fall back to showing an abbreviated object name

Some callers may find it useful if "git describe" always gave back a
string that can be used as a shorter name for a commit object, rather than
checking its exit status (while squelching its error message, which could
potentially talk about more grave errors that should not be squelched) and
implementing a fallback themselves.

This teaches describe/name-rev a new option, --always, to use an
abbreviated object name when no tags or refs to use is found.

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