Opening a symlinked file showed the whole file as changed in the git
gutter/diff. Git stores only the symlink's target path as that entry's
blob, while the buffer holds the resolved target's contents, so diffing
the two is meaningless.

We now skip loading a git diff base whenever a buffer's worktree entry
is a symlink i.e. its `canonical_path` is set.

Release Notes:

- Fixed symlinked files showing the entire file as changed in the git
diff/gutter.