rebase tests: repeat some tests using the merge backend instead of am

In order to ensure the merge/interactive backend gets similar coverage
to the am one, add some tests for cases where previously only the am
backend was tested.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t5520-pull.sh

@@ -277,14 +277,27 @@ test_expect_success '--rebase' '
 	test_cmp expect actual
 '
 
-test_expect_success '--rebase fast forward' '
+test_expect_success '--rebase (merge) fast forward' '
 	git reset --hard before-rebase &&
 	git checkout -b ff &&
 	echo another modification >file &&
 	git commit -m third file &&
 
 	git checkout to-rebase &&
-	git pull --rebase . ff &&
+	git -c rebase.backend=merge pull --rebase . ff &&
+	test_cmp_rev HEAD ff &&
+
+	# The above only validates the result.  Did we actually bypass rebase?
+	git reflog -1 >reflog.actual &&
+	sed "s/^[0-9a-f][0-9a-f]*/OBJID/" reflog.actual >reflog.fuzzy &&
+	echo "OBJID HEAD@{0}: pull --rebase . ff: Fast-forward" >reflog.expected &&
+	test_cmp reflog.expected reflog.fuzzy
+'
+
+test_expect_success '--rebase (am) fast forward' '
+	git reset --hard before-rebase &&
+
+	git -c rebase.backend=am pull --rebase . ff &&
 	test_cmp_rev HEAD ff &&
 
 	# The above only validates the result.  Did we actually bypass rebase?

t/t6047-diff3-conflict-markers.sh

@@ -186,15 +186,24 @@ test_expect_success 'check multiple merge bases' '
 	)
 '
 
-test_expect_success 'rebase describes fake ancestor base' '
+test_expect_success 'rebase --merge describes parent of commit being picked' '
 	test_create_repo rebase &&
 	(
 		cd rebase &&
 		test_commit base file &&
 		test_commit master file &&
 		git checkout -b side HEAD^ &&
 		test_commit side file &&
-		test_must_fail git -c merge.conflictstyle=diff3 rebase master &&
+		test_must_fail git -c merge.conflictstyle=diff3 rebase --merge master &&
+		grep "||||||| parent of" file
+	)
+'
+
+test_expect_success 'rebase --am describes fake ancestor base' '
+	(
+		cd rebase &&
+		git rebase --abort &&
+		test_must_fail git -c merge.conflictstyle=diff3 rebase --am master &&
 		grep "||||||| constructed merge base" file
 	)
 '