t3404: test two "preserve merges with -p" cases

There are two cases for preserving merges:

  1. The merge base is outside the trunk that is to be rebased.
     Then commits of those other parents must not be picked.

  2. The merge base is inside the trunk that is to be rebased.
     Then all the commits related to that merge must be picked
     and the merge must be redone.

The "preserve merges with -p" test case tested for case 1 only.
This patch adds case 2.

Signed-off-by: Stephan Beyer <s-beyer@gmx.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t3404-rebase-interactive.sh

@@ -211,7 +211,7 @@ test_expect_success 'preserve merges with -p' '
 	git add unrelated-file &&
 	test_tick &&
 	git commit -m "unrelated" &&
-	git checkout -b to-be-rebased master &&
+	git checkout -b another-branch master &&
 	echo B > file1 &&
 	test_tick &&
 	git commit -m J file1 &&
@@ -220,12 +220,28 @@ test_expect_success 'preserve merges with -p' '
 	echo C > file1 &&
 	test_tick &&
 	git commit -m K file1 &&
+	echo D > file1 &&
+	test_tick &&
+	git commit -m L1 file1 &&
+	git checkout HEAD^ &&
+	echo 1 > unrelated-file &&
+	test_tick &&
+	git commit -m L2 unrelated-file &&
+	test_tick &&
+	git merge another-branch &&
+	echo E > file1 &&
+	test_tick &&
+	git commit -m M file1 &&
+	git checkout -b to-be-rebased &&
 	test_tick &&
 	git rebase -i -p --onto branch1 master &&
-	test $(git rev-parse HEAD^^2) = $(git rev-parse to-be-preserved) &&
-	test $(git rev-parse HEAD~3) = $(git rev-parse branch1) &&
-	test $(git show HEAD:file1) = C &&
-	test $(git show HEAD~2:file1) = B
+	test $(git rev-parse HEAD~6) = $(git rev-parse branch1) &&
+	test $(git rev-parse HEAD~4^2) = $(git rev-parse to-be-preserved) &&
+	test $(git rev-parse HEAD^^2^) = $(git rev-parse HEAD^^^) &&
+	test $(git show HEAD~5:file1) = B &&
+	test $(git show HEAD~3:file1) = C &&
+	test $(git show HEAD:file1) = E &&
+	test $(git show HEAD:unrelated-file) = 1
 '
 
 test_expect_success '--continue tries to commit' '