diff: do not reuse_worktree_file for submodules

The GIT_EXTERNAL_DIFF calling code attempts to reuse existing worktree
files for the worktree side of diffs, for performance reasons.
However, that code also tries to do the same with submodules.  This
results in calls to $GIT_EXTERNAL_DIFF where the old-file is a file of
the form "Submodule commit $sha1", but the new-file is a directory in
the worktree.

Fix it by never reusing a worktree "file" in the submodule case.

Reported-by: Grégory Pakosz <gregory.pakosz@gmail.com>
Signed-off-by: Thomas Rast <tr@thomasrast.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff.c

@@ -2842,8 +2842,9 @@ static struct diff_tempfile *prepare_temp_file(const char *name,
 		remove_tempfile_installed = 1;
 	}
 
-	if (!one->sha1_valid ||
-	    reuse_worktree_file(name, one->sha1, 1)) {
+	if (!S_ISGITLINK(one->mode) &&
+	    (!one->sha1_valid ||
+	     reuse_worktree_file(name, one->sha1, 1))) {
 		struct stat st;
 		if (lstat(name, &st) < 0) {
 			if (errno == ENOENT)

t/t4020-diff-external.sh

@@ -213,17 +213,45 @@ keep_only_cr () {
 }
 
 test_expect_success 'external diff with autocrlf = true' '
-	git config core.autocrlf true &&
+	test_config core.autocrlf true &&
 	GIT_EXTERNAL_DIFF=./fake-diff.sh git diff &&
 	test $(wc -l < crlfed.txt) = $(cat crlfed.txt | keep_only_cr | wc -c)
 '
 
 test_expect_success 'diff --cached' '
+	test_config core.autocrlf true &&
 	git add file &&
 	git update-index --assume-unchanged file &&
 	echo second >file &&
 	git diff --cached >actual &&
 	test_cmp "$TEST_DIRECTORY"/t4020/diff.NUL actual
 '
 
+test_expect_success 'clean up crlf leftovers' '
+	git update-index --no-assume-unchanged file &&
+	rm -f file* &&
+	git reset --hard
+'
+
+test_expect_success 'submodule diff' '
+	git init sub &&
+	( cd sub && test_commit sub1 ) &&
+	git add sub &&
+	test_tick &&
+	git commit -m "add submodule" &&
+	( cd sub && test_commit sub2 ) &&
+	write_script gather_pre_post.sh <<-\EOF &&
+	echo "$1 $4" # path, mode
+	cat "$2" # old file
+	cat "$5" # new file
+	EOF
+	GIT_EXTERNAL_DIFF=./gather_pre_post.sh git diff >actual &&
+	cat >expected <<-EOF &&
+	sub 160000
+	Subproject commit $(git rev-parse HEAD:sub)
+	Subproject commit $(cd sub && git rev-parse HEAD)
+	EOF
+	test_cmp expected actual
+'
+
 test_done