diff: handle --no-abbrev in no-index case

diff: handle --no-abbrev in no-index case

There are two different places where the --no-abbrev option is parsed,
and two different places where SHA-1s are abbreviated. We normally parse
--no-abbrev with setup_revisions(), but in the no-index case, "git diff"
calls diff_opt_parse() directly, and diff_opt_parse() didn't handle
--no-abbrev until now. (It did handle --abbrev, however.) We normally
abbreviate SHA-1s with find_unique_abbrev(), but commit 4f03666 ("diff:
handle sha1 abbreviations outside of repository, 2016-10-20) recently
introduced a special case when you run "git diff" outside of a
repository.

setup_revisions() does also call diff_opt_parse(), but not for --abbrev
or --no-abbrev, which it handles itself. setup_revisions() sets
rev_info->abbrev, and later copies that to diff_options->abbrev. It
handles --no-abbrev by setting abbrev to zero. (This change doesn't
touch that.)

Setting abbrev to zero was broken in the outside-of-a-repository special
case, which until now resulted in a truly zero-length SHA-1, rather than
taking zero to mean do not abbreviate. The only way to trigger this bug,
however, was by running "git diff --raw" without either the --abbrev or
--no-abbrev options, because 1) without --raw it doesn't respect abbrev
(which is bizarre, but has been that way forever), 2) we silently clamp
--abbrev=0 to MINIMUM_ABBREV, and 3) --no-abbrev wasn't handled until
now.

The outside-of-a-repository case is one of three no-index cases. The
other two are when one of the files you're comparing is outside of the
repository you're in, and the --no-index option.

Signed-off-by: Jack Bates <jack@nottheoilrig.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>