[PATCH] read-cache: disable renames in add_files_to_cache

[blindmansion <blindmansion@gmail.com>]

add_files_to_cache() refreshes the index from worktree changes and does
not need rename detection. When unmerged entries and a deleted stage-0
path are present together, rename detection can pair them and rewrite an
unmerged diff pair to point at the deleted path.

That later makes "git commit -a" and "git add -u" try to stat the
deleted path and die with "unable to stat". Disable rename detection in
this callback-driven staging path and add a regression test covering the
crash.
---
 read-cache.c          |  1 +
 t/t2200-add-update.sh | 39 +++++++++++++++++++++++++++++++++++++++
 2 files changed, 40 insertions(+)

diff --git a/read-cache.c b/read-cache.c
index 5049f9b..d938abc 100644
--- a/read-cache.c
+++ b/read-cache.c
@@ -4049,6 +4049,7 @@ int add_files_to_cache(struct repository *repo, const char *prefix,
 	rev.diffopt.format_callback = update_callback;
 	rev.diffopt.format_callback_data = &data;
 	rev.diffopt.flags.override_submodule_config = 1;
+	rev.diffopt.detect_rename = 0; /* staging worktree changes does not need renames */
 	rev.max_count = 0; /* do not compare unmerged paths with stage #2 */
 
 	/*
diff --git a/t/t2200-add-update.sh b/t/t2200-add-update.sh
index 06e83d3..56c7e55 100755
--- a/t/t2200-add-update.sh
+++ b/t/t2200-add-update.sh
@@ -200,6 +200,45 @@ test_expect_success 'add -u resolves unmerged paths' '
 	test_cmp expect actual
 '
 
+test_expect_success 'add -u avoids rename pairing on unmerged paths' '
+	test_create_repo rename-crash &&
+	(
+		cd rename-crash &&
+		test_seq 1 100 |
+		sed "s/.*/line &: shared content that is identical across both files/" >conflict.txt &&
+		cp conflict.txt bystander.txt &&
+		git add conflict.txt bystander.txt &&
+		git commit -m "initial: two files with identical content" &&
+		main_branch=$(git symbolic-ref --short HEAD) &&
+		git checkout -b feature &&
+		perl -pe '\''s/^line 50:.*/line 50: FEATURE BRANCH CHANGE/'\'' \
+			conflict.txt >conflict.txt.tmp &&
+		mv conflict.txt.tmp conflict.txt &&
+		git add conflict.txt &&
+		git commit -m "feature: modify line 50" &&
+		git checkout "$main_branch" &&
+		perl -pe '\''s/^line 50:.*/line 50: MAIN BRANCH CHANGE/'\'' \
+			conflict.txt >conflict.txt.tmp &&
+		mv conflict.txt.tmp conflict.txt &&
+		git add conflict.txt &&
+		git commit -m "main: modify line 50 differently" &&
+		test_must_fail git merge feature &&
+		rm bystander.txt &&
+		git add -u >out 2>err &&
+		test_must_be_empty out &&
+		test_must_be_empty err &&
+		git ls-files -u >actual &&
+		test_must_be_empty actual &&
+		git ls-files bystander.txt >actual &&
+		test_must_be_empty actual &&
+		echo conflict.txt >expect &&
+		git ls-files conflict.txt >actual &&
+		test_cmp expect actual &&
+		git diff-files --name-only >actual &&
+		test_must_be_empty actual
+	)
+'
+
 test_expect_success '"add -u non-existent" should fail' '
 	test_must_fail git add -u non-existent &&
 	git ls-files >actual &&
-- 
2.53.0