t7800: fix racy "difftool --dir-diff syncs worktree" test

The "difftool --dir-diff syncs worktree without unstaged change" test
fails intermittently, particularly on Windows CI.

The test modifies a file in difftool's temp directory via an extcmd
script and expects the change to be synced back to the worktree. The
sync-back detection relies on git's change detection mechanisms.

The root cause is that the original file content and the replacement
content have identical sizes:

  - Original: "main\ntest\na\n" = 12 bytes
  - New:      "new content\n"   = 12 bytes

When difftool creates the temporary index (wtindex), the cache entries
have sd_size = 0 (zero-initialized via make_cache_entry with no
refresh). Git's ie_modified() is designed to handle this by calling
ce_modified_check_fs() for content hashing when sd_size is 0.

However, Windows has known filesystem issues that may cause this to
fail intermittently:

 - UNRELIABLE_FSTAT: Windows fstat() on open files may not return the
   same information as lstat() after close (config.mak.uname:506)

 - NTFS timestamp issues: The racy-git documentation notes that NTFS
   is "still broken" regarding timestamp granularity between in-core
   and on-disk representations (Documentation/technical/racy-git.adoc)

 - Attribute caching: Windows GetFileAttributesExW may cache results

Fix this by changing the replacement content to "modified content\n"
(17 bytes), ensuring the change is detected at the earliest size
comparison in match_stat_data(), bypassing any platform-specific edge
cases in the more complex code paths.

Note: Other tests with same-size file patterns (t0010-racy-git.sh,
t2200-add-update.sh, t1701-racy-split-index.sh) are not vulnerable
because they use normal Git index operations with proper racy git
detection. The difftool case is unique due to its ephemeral wtindex
created via make_cache_entry() without full stat refresh.

Signed-off-by: Paul Tarjan <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: ptarjan

t/t7800-difftool.sh

@@ -647,21 +647,21 @@ test_expect_success SYMLINKS 'difftool --dir-diff --symlinks without unstaged ch
 '
 
 write_script modify-right-file <<\EOF
-echo "new content" >"$2/file"
+echo "modified content" >"$2/file"
 EOF
 
 run_dir_diff_test 'difftool --dir-diff syncs worktree with unstaged change' '
 	test_when_finished git reset --hard &&
 	echo "orig content" >file &&
 	git difftool -d $symlinks --extcmd "$PWD/modify-right-file" branch &&
-	echo "new content" >expect &&
+	echo "modified content" >expect &&
 	test_cmp expect file
 '
 
 run_dir_diff_test 'difftool --dir-diff syncs worktree without unstaged change' '
 	test_when_finished git reset --hard &&
 	git difftool -d $symlinks --extcmd "$PWD/modify-right-file" branch &&
-	echo "new content" >expect &&
+	echo "modified content" >expect &&
 	test_cmp expect file
 '