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>

builtin-rev-list.c

@@ -313,7 +313,7 @@ int cmd_rev_list(int argc, const char **argv, const char *prefix)
 
 	git_config(git_default_config, NULL);
 	init_revisions(&revs, prefix);
-	revs.abbrev = 0;
+	revs.abbrev = DEFAULT_ABBREV;
 	revs.commit_format = CMIT_FMT_UNSPECIFIED;
 	argc = setup_revisions(argc, argv, &revs, NULL);