t6423: add a testcase causing a failed assertion in process_renames

If one side of history renames a directory A/ -> B/, and the other side
of history adds new files to A/, then directory rename detection notices
and moves or suggests moving those new files to B/.  A similar thing is
done for paths renamed into A/, causing them to be transitively renamed
into B/.  But, if the file originally came from B/, then this can end up
causing a file to be renamed back to itself.  merge-ort crashes under
this special case, due to a slightly overzealous assertion:

    git: merge-ort.c:3051: process_renames: Assertion `source_deleted || oldinfo->filemask & old_sidemask' failed.
    Aborted (core dumped)

Add a testcase demonstrating this.

Signed-off-by: Dmitry Goncharov <[email protected]>
[en: Instead of adding a new testsuite, place it near similar tests in
 t6423, adjusting to match the style of those tests.  Tweak the commit
 message to not repeat the entire testcase, but just describe the bug.
 Also update the line number in the error message.]
Signed-off-by: Elijah Newren <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: gtkiller

t/t6423-merge-rename-directories.sh

@@ -5363,6 +5363,47 @@ test_expect_merge_algorithm failure success '12m: Change parent of renamed-dir t
 	)
 '
 
+test_setup_12n () {
+	git init 12n &&
+	(
+		cd 12n &&
+
+		mkdir tools &&
+		echo hello >tools/hello &&
+		git add tools/hello &&
+		git commit -m "O" &&
+
+		git branch O &&
+		git branch A &&
+		git branch B &&
+
+		git switch A &&
+		echo world >world &&
+		git add world &&
+		git commit -q world -m 'Add world' &&
+
+		git mv world tools/world &&
+		git commit -m "Move world into tools/" &&
+
+		git switch B &&
+		git mv tools/hello hello &&
+		git commit -m "Move hello from tools/ to toplevel"
+	)
+}
+
+test_expect_failure '12n: Directory rename transitively makes rename back to self' '
+	test_setup_12n &&
+	(
+		cd 12n &&
+
+		git checkout -q B^0 &&
+
+		test_must_fail git cherry-pick A^0 >out &&
+		grep "CONFLICT (file location).*should perhaps be moved" out
+	)
+'
+
+
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 # SECTION 13: Checking informational and conflict messages
 #