rev-list: refuse --first-parent combined with --bisect

rev-list --bisect is used by git bisect, but never together with
--first-parent. Because rev-list --bisect together with --first-parent
is not handled currently, and even leads to segfaults, refuse to use
both options together.

Because this is not supported, it makes little sense to use git log
--bisect --first parent either, because refs/heads/bad is not limited to
the first parent chain.

Helped-by: Junio C. Hamano <gitster@pobox.com>
Helped-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Kevin Daudt <me@ikke.info>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/rev-list-options.txt

@@ -119,7 +119,8 @@ parents) and `--max-parents=-1` (negative numbers denote no upper limit).
 	because merges into a topic branch tend to be only about
 	adjusting to updated upstream from time to time, and
 	this option allows you to ignore the individual commits
-	brought in to your history by such a merge.
+	brought in to your history by such a merge. Cannot be
+	combined with --bisect.
 
 --not::
 	Reverses the meaning of the '{caret}' prefix (or lack thereof)
@@ -177,7 +178,7 @@ ifndef::git-rev-list[]
 	Pretend as if the bad bisection ref `refs/bisect/bad`
 	was listed and as if it was followed by `--not` and the good
 	bisection refs `refs/bisect/good-*` on the command
-	line.
+	line. Cannot be combined with --first-parent.
 endif::git-rev-list[]
 
 --stdin::
@@ -558,7 +559,7 @@ outputs 'midpoint', the output of the two commands
 would be of roughly the same length.  Finding the change which
 introduces a regression is thus reduced to a binary search: repeatedly
 generate and test new 'midpoint's until the commit chain is of length
-one.
+one. Cannot be combined with --first-parent.
 
 --bisect-vars::
 	This calculates the same as `--bisect`, except that refs in

revision.c

@@ -2273,6 +2273,9 @@ int setup_revisions(int argc, const char **argv, struct rev_info *revs, struct s
 	if (!revs->reflog_info && revs->grep_filter.use_reflog_filter)
 		die("cannot use --grep-reflog without --walk-reflogs");
 
+	if (revs->first_parent_only && revs->bisect)
+		die(_("--first-parent is incompatible with --bisect"));
+
 	return left;
 }
 

t/t6000-rev-list-misc.sh

@@ -73,4 +73,8 @@ test_expect_success 'symleft flag bit is propagated down from tag' '
 	test_cmp expect actual
 '
 
+test_expect_success '--bisect and --first-parent can not be combined' '
+	test_must_fail git rev-list --bisect --first-parent HEAD
+'
+
 test_done