rev-list: use default abbrev length when abbrev-commit is in effect

rev-list: use default abbrev length when abbrev-commit is in effect

Currently, rev-list has a default of "0" for abbrev which means that
switching on abbreviations with --abbrev-commit has no visible effect,
even though the option is documented.

Set abbrev to DEFAULT_ABBREV so that --abbrev-commit has the same effect
as for log.

Reported-by: Eli Barzilay <eli@barzilay.org>
Signed-off-by: Michael J Gruber <git@drmicha.warpmail.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>