mergetool: support delete/delete conflicts

If two branches each move a file into different directories then
mergetool will fail because it assumes that the file being merged, and
its parent directory, are present in the worktree.

Create the merge file's parent directory to allow using the
deleted base version of the file for merge resolution when
encountering a delete/delete conflict.

The end result is that a delete/delete conflict is presented for the
user to resolve.

Reported-by: Joe Einertson <joe@kidblog.org>
Signed-off-by: David Aguilar <davvid@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-mergetool.sh

@@ -282,8 +282,14 @@ merge_file () {
 		return
 	fi
 
-	mv -- "$MERGED" "$BACKUP"
-	cp -- "$BACKUP" "$MERGED"
+	if test -f "$MERGED"
+	then
+		mv -- "$MERGED" "$BACKUP"
+		cp -- "$BACKUP" "$MERGED"
+	fi
+	# Create a parent directory to handle delete/delete conflicts
+	# where the base's directory no longer exists.
+	mkdir -p "$(dirname "$MERGED")"
 
 	checkout_staged_file 1 "$MERGED" "$BASE"
 	checkout_staged_file 2 "$MERGED" "$LOCAL"
@@ -295,7 +301,9 @@ merge_file () {
 		describe_file "$local_mode" "local" "$LOCAL"
 		describe_file "$remote_mode" "remote" "$REMOTE"
 		resolve_deleted_merge
-		return
+		status=$?
+		rmdir -p "$(dirname "$MERGED")" 2>/dev/null
+		return $status
 	fi
 
 	if is_symlink "$local_mode" || is_symlink "$remote_mode"

t/t7610-mergetool.sh

@@ -243,6 +243,45 @@ test_expect_success 'mergetool takes partial path' '
 	git reset --hard
 '
 
+test_expect_success 'mergetool delete/delete conflict' '
+	git checkout -b delete-base branch1 &&
+	mkdir -p a/a &&
+	(echo one; echo two; echo 3; echo 4) >a/a/file.txt &&
+	git add a/a/file.txt &&
+	git commit -m"base file" &&
+	git checkout -b move-to-b delete-base &&
+	mkdir -p b/b &&
+	git mv a/a/file.txt b/b/file.txt &&
+	(echo one; echo two; echo 4) >b/b/file.txt &&
+	git commit -a -m"move to b" &&
+	git checkout -b move-to-c delete-base &&
+	mkdir -p c/c &&
+	git mv a/a/file.txt c/c/file.txt &&
+	(echo one; echo two; echo 3) >c/c/file.txt &&
+	git commit -a -m"move to c" &&
+	test_must_fail git merge move-to-b &&
+	echo d | git mergetool a/a/file.txt &&
+	! test -f a/a/file.txt &&
+	git reset --hard HEAD &&
+	test_must_fail git merge move-to-b &&
+	echo m | git mergetool a/a/file.txt &&
+	test -f b/b/file.txt &&
+	git reset --hard HEAD &&
+	test_must_fail git merge move-to-b &&
+	! echo a | git mergetool a/a/file.txt &&
+	! test -f a/a/file.txt &&
+	git reset --hard HEAD
+'
+
+test_expect_success 'mergetool produces no errors when keepBackup is used' '
+	test_config mergetool.keepBackup true &&
+	test_must_fail git merge move-to-b &&
+	: >expect &&
+	echo d | git mergetool a/a/file.txt 2>actual &&
+	test_cmp expect actual &&
+	git reset --hard HEAD
+'
+
 test_expect_success 'deleted vs modified submodule' '
 	git checkout -b test6 branch1 &&
 	git submodule update -N &&