merge-recursive: don't segfault while handling rename clashes

When a branch moves A to B while the other branch created B (or moved C to
B), the code tried to rename one of them to B~something to preserve both
versions, and failed to register temporary resolution for the original
path B at stage#0 during virtual ancestor computation.  This left the
index in unmerged state and caused a segfault.

A better solution is to merge these two versions of B's in place and use
the (potentially conflicting) result as the intermediate merge result in
the virtual ancestor.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t6036-recursive-corner-cases.sh

@@ -0,0 +1,55 @@
+#!/bin/sh
+
+test_description='recursive merge corner cases'
+
+. ./test-lib.sh
+
+#
+#  L1  L2
+#   o---o
+#  / \ / \
+# o   X   ?
+#  \ / \ /
+#   o---o
+#  R1  R2
+#
+
+test_expect_success setup '
+	ten="0 1 2 3 4 5 6 7 8 9"
+	for i in $ten
+	do
+		echo line $i in a sample file
+	done >one &&
+	for i in $ten
+	do
+		echo line $i in another sample file
+	done >two &&
+	git add one two &&
+	test_tick && git commit -m initial &&
+
+	git branch L1 &&
+	git checkout -b R1 &&
+	git mv one three &&
+	test_tick && git commit -m R1 &&
+
+	git checkout L1 &&
+	git mv two three &&
+	test_tick && git commit -m L1 &&
+
+	git checkout L1^0 &&
+	test_tick && git merge -s ours R1 &&
+	git tag L2 &&
+
+	git checkout R1^0 &&
+	test_tick && git merge -s ours L1 &&
+	git tag R2
+'
+
+test_expect_success merge '
+	git reset --hard &&
+	git checkout L2^0 &&
+
+	test_must_fail git merge -s recursive R2^0
+'
+
+test_done