git: Don't load shallow HEAD text of symlinks (#25058)

For symlinks, return `None` from `load_committed_text` as we do from
`load_index_text` ever since #10037.

Release Notes:

- Fixed diff hunks appearing in unchanged symlinked files
76f501af71Cole Miller committed on 2/18/2025, 3:18:23 PM· committed by GitHubparent9ef0501
1 file changedLine totals unavailable