rebase --preserve-merges: ignore "merge.log" config

When "merge.log" config is set, "rebase --preserve-merges" will add
the log lines to the message of the rebased merge commit.  A rebase
should not modify a commit message automatically.

Teach "git-rebase" to ignore that configuration by passing
"--no-log" to the git-merge call.

Signed-off-by: Ralf Thielow <ralf.thielow@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-rebase--interactive.sh

@@ -352,8 +352,9 @@ pick_one_preserving_merges () {
 			msg_content="$(commit_message $sha1)"
 			# No point in merging the first parent, that's HEAD
 			new_parents=${new_parents# $first_parent}
+			merge_args="--no-log --no-ff"
 			if ! do_with_author output eval \
-			'git merge --no-ff $strategy_args -m "$msg_content" $new_parents'
+			'git merge $merge_args $strategy_args -m "$msg_content" $new_parents'
 			then
 				printf "%s\n" "$msg_content" > "$GIT_DIR"/MERGE_MSG
 				die_with_patch $sha1 "Error redoing merge $sha1"

t/t3409-rebase-preserve-merges.sh

@@ -28,6 +28,8 @@ export GIT_AUTHOR_EMAIL
 #     \--A3    <-- topic2
 #      \
 #       B2     <-- origin/topic
+#
+# Clone 4 (same as Clone 3)
 
 test_expect_success 'setup for merge-preserving rebase' \
 	'echo First > A &&
@@ -64,6 +66,16 @@ test_expect_success 'setup for merge-preserving rebase' \
 		git merge --no-ff topic2
 	) &&
 
+	git clone ./. clone4 &&
+	(
+		cd clone4 &&
+		git checkout -b topic2 origin/topic &&
+		echo Sixth > A &&
+		git commit -a -m "Modify A3" &&
+		git checkout -b topic origin/topic &&
+		git merge --no-ff topic2
+	) &&
+
 	git checkout topic &&
 	echo Fourth >> B &&
 	git commit -a -m "Modify B2"
@@ -96,4 +108,15 @@ test_expect_success 'rebase -p preserves no-ff merges' '
 	)
 '
 
+test_expect_success 'rebase -p ignores merge.log config' '
+	(
+	cd clone4 &&
+	git fetch &&
+	git -c merge.log=1 rebase -p origin/topic &&
+	echo >expected &&
+	git log --format="%b" -1 >current &&
+	test_cmp expected current
+	)
+'
+
 test_done