[PATCH] simplify-merges: drop merge from irrelevant side branch

[Junio C Hamano <gitster@pobox.com>]

The merge simplification rule stated in 6546b59 (revision traversal:
show full history with merge simplification, 2008-07-31) still
treated merge commits too specially.  Namely, in a history with this
shape:

	---o---o---M
	          /
         x---x---x

where three 'x' were on a history completely unrelated to the main
history 'o' and do not touch any of the paths we are following, we
still said that after simplifying all of the parents of M, 'x'
(which is the leftmost 'x' that rightmost 'x simplifies down to) and
'o' (which would be the last commit on the main history that touches
the paths we are following) are independent from each other, and
both need to be kept.

That is incorrect; when the side branch 'x' never touches the paths,
it should be removed to allow M to simplify down to the last commit
on the main history that touches the paths.

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

 * The old test vector had a history ending at I; the updates to it
   adds an unrelated side branch rooted at J, merge of I and J which
   is K, and then an extra commit L on top.

	A---(some merge mess)---I---K---L
                                   /
                                  J

 revision.c                   | 19 +++++++++++++++++++
 t/t6012-rev-list-simplify.sh | 26 +++++++++++++++++++++-----
 2 files changed, 40 insertions(+), 5 deletions(-)

diff --git a/revision.c b/revision.c
index 33cb207..532611b 100644
--- a/revision.c
+++ b/revision.c
@@ -1424,6 +1424,22 @@ static struct merge_simplify_state *locate_simplify_state(struct rev_info *revs,
 	return st;
 }
 
+static void remove_treesame_parents(struct commit *commit)
+{
+	struct commit_list **pp, *p;
+
+	pp = &commit->parents;
+	while ((p = *pp) != NULL) {
+		struct commit *parent = p->item;
+		if (parent->object.flags & TREESAME) {
+			*pp = p->next;
+			free(p);
+			continue;
+		}
+		pp = &p->next;
+	}
+}
+
 static struct commit_list **simplify_one(struct rev_info *revs, struct commit *commit, struct commit_list **tail)
 {
 	struct commit_list *p;
@@ -1469,6 +1485,9 @@ static struct commit_list **simplify_one(struct rev_info *revs, struct commit *c
 		pst = locate_simplify_state(revs, p->item);
 		p->item = pst->simplified;
 	}
+
+	remove_treesame_parents(commit);
+
 	cnt = remove_duplicate_parents(commit);
 
 	/*
diff --git a/t/t6012-rev-list-simplify.sh b/t/t6012-rev-list-simplify.sh
index 510bb96..d6d79c4 100755
--- a/t/t6012-rev-list-simplify.sh
+++ b/t/t6012-rev-list-simplify.sh
@@ -59,7 +59,23 @@ test_expect_success setup '
 
 	echo "Final change" >file &&
 	test_tick && git commit -a -m "Final change" &&
-	note I
+	note I &&
+
+	git symbolic-ref HEAD refs/heads/unrelated &&
+	git rm -f "*" &&
+	echo "Unrelated branch" >side &&
+	git add side &&
+	test_tick && git commit -m "Side root" &&
+	note J &&
+
+	git checkout master &&
+	test_tick && git merge -m "Coolest" unrelated &&
+	note K &&
+
+	echo "Immaterial" >elif &&
+	git add elif &&
+	test_tick && git commit -m "Last" &&
+	note L
 '
 
 FMT='tformat:%P 	%H | %s'
@@ -82,10 +98,10 @@ check_result () {
 	'
 }
 
-check_result 'I H G F E D C B A' --full-history
-check_result 'I H E C B A' --full-history -- file
-check_result 'I H E C B A' --full-history --topo-order -- file
-check_result 'I H E C B A' --full-history --date-order -- file
+check_result 'L K J I H G F E D C B A' --full-history
+check_result 'K I H E C B A' --full-history -- file
+check_result 'K I H E C B A' --full-history --topo-order -- file
+check_result 'K I H E C B A' --full-history --date-order -- file
 check_result 'I E C B A' --simplify-merges -- file
 check_result 'I B A' -- file
 check_result 'I B A' --topo-order -- file
-- 
1.8.1.1.431.g469ab37