t3401: use test_commit in setup

Simplify t3401 by using test_commit in the setup. This lets us refer
to commits using their tags and there is no longer a need to create
the branch my-topic-branch-merge. Also, the branch master-merge points
to the same commit as master (even before this change), so that branch
does not need to be created either.

While at it, replace "test ! -d" by "test_path_is_missing".

Signed-off-by: Martin von Zweigbergk <martin.von.zweigbergk@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: Martin von Zweigbergk

t/t3401-rebase-partial.sh

@@ -12,32 +12,17 @@ local branch.
 . ./test-lib.sh
 
 test_expect_success 'prepare repository with topic branch' '
-	echo First > A &&
-	git update-index --add A &&
-	git commit -m "Add A." &&
-
+	test_commit A &&
 	git checkout -b my-topic-branch &&
-
-	echo Second > B &&
-	git update-index --add B &&
-	git commit -m "Add B." &&
-
-	echo AnotherSecond > C &&
-	git update-index --add C &&
-	git commit -m "Add C." &&
-
+	test_commit B &&
+	test_commit C &&
 	git checkout -f master &&
-
-	echo Third >> A &&
-	git update-index A &&
-	git commit -m "Modify A."
+	test_commit A2 A.t
 '
 
 test_expect_success 'pick top patch from topic branch into master' '
-	git cherry-pick my-topic-branch^0 &&
-	git checkout -f my-topic-branch &&
-	git branch master-merge master &&
-	git branch my-topic-branch-merge my-topic-branch
+	git cherry-pick C &&
+	git checkout -f my-topic-branch
 '
 
 test_debug '
@@ -48,13 +33,13 @@ test_debug '
 
 test_expect_success 'rebase topic branch against new master and check git am did not get halted' '
 	git rebase master &&
-	test ! -d .git/rebase-apply
+	test_path_is_missing .git/rebase-apply
 '
 
 test_expect_success 'rebase --merge topic branch that was partially merged upstream' '
-	git checkout -f my-topic-branch-merge &&
-	git rebase --merge master-merge &&
-	test ! -d .git/rebase-merge
+	git reset --hard C &&
+	git rebase --merge master &&
+	test_path_is_missing .git/rebase-merge
 '
 
 test_done