submodule: Demonstrate known breakage during recursive merge

Since commit 68d03e4 (Implement automatic fast-forward merge for
submodules, 2010-07-07) we try to suggest submodule commits that resolve
a conflict.  Consider a true recursive merge case

    b---bc
   / \ /
  o   X
   \ / \
    c---cb

in which the two heads themselves (bc,cb) had resolved a submodule
conflict (i.e. reference different commits than their parents).  The
submodule merge search runs during the temporary merge of the two merge
bases (b,c) and prints out a suggestion that is not meaningful to the
user.  Then during the main merge the submodule merge search runs again
but dies with the message

  fatal: --ancestry-path given but there are no bottom commits

while trying to enumerate candidates.  Demonstrate this known breakage
with a new test in t7405-submodule-merge covering the case.

Signed-off-by: Brad King <brad.king@kitware.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t7405-submodule-merge.sh

@@ -228,4 +228,55 @@ test_expect_success 'merging with a modify/modify conflict between merge bases'
 	git merge d
 '
 
+# canonical criss-cross history in top and submodule
+test_expect_success 'setup for recursive merge with submodule' '
+	mkdir merge-recursive &&
+	(cd merge-recursive &&
+	 git init &&
+	 mkdir sub &&
+	 (cd sub &&
+	  git init &&
+	  test_commit a &&
+	  git checkout -b sub-b master &&
+	  test_commit b &&
+	  git checkout -b sub-c master &&
+	  test_commit c &&
+	  git checkout -b sub-bc sub-b &&
+	  git merge sub-c &&
+	  git checkout -b sub-cb sub-c &&
+	  git merge sub-b &&
+	  git checkout master) &&
+	 git add sub &&
+	 git commit -m a &&
+	 git checkout -b top-b master &&
+	 (cd sub && git checkout sub-b) &&
+	 git add sub &&
+	 git commit -m b &&
+	 git checkout -b top-c master &&
+	 (cd sub && git checkout sub-c) &&
+	 git add sub &&
+	 git commit -m c &&
+	 git checkout -b top-bc top-b &&
+	 git merge -s ours --no-commit top-c &&
+	 (cd sub && git checkout sub-bc) &&
+	 git add sub &&
+	 git commit -m bc &&
+	 git checkout -b top-cb top-c &&
+	 git merge -s ours --no-commit top-b &&
+	 (cd sub && git checkout sub-cb) &&
+	 git add sub &&
+	 git commit -m cb)
+'
+
+# merge should leave submodule unmerged in index
+test_expect_failure 'recursive merge with submodule' '
+	(cd merge-recursive &&
+	 test_must_fail git merge top-bc &&
+	 echo "160000 $(git rev-parse top-cb:sub) 2	sub" > expect2 &&
+	 echo "160000 $(git rev-parse top-bc:sub) 3	sub" > expect3 &&
+	 git ls-files -u > actual &&
+	 grep "$(cat expect2)" actual > /dev/null &&
+	 grep "$(cat expect3)" actual > /dev/null)
+'
+
 test_done