cherry-pick: demonstrate a segmentation fault

In #952, a complicated
scenario was described that leads to a segmentation fault in
cherry-pick.

It boils down to a certain code path involving a renamed file that is
dirty, for which `refresh_cache_entry()` returns `NULL`, and that
`NULL` not being handled properly.

Signed-off-by: Johannes Schindelin <[email protected]>

Author: dscho

t/t3501-revert-cherry-pick.sh

@@ -141,4 +141,16 @@ test_expect_success 'cherry-pick "-" works with arguments' '
 	test_cmp expect actual
 '
 
+test_expect_failure 'cherry-pick works with dirty renamed file' '
+	test_commit to-rename &&
+	git checkout -b unrelated &&
+	test_commit unrelated &&
+	git checkout @{-1} &&
+	git mv to-rename.t renamed &&
+	test_tick &&
+	git commit -m renamed &&
+	echo modified >renamed &&
+	git cherry-pick refs/heads/unrelated
+'
+
 test_done