Make RealFs::metadata not error on recursive/looped symlinks

[sourcefrog]

In the Zed tree, crates/eval_utils/LICENSE-GPL is a symlink to itself:

drwxrwxr-x   3 mbp mbp  4096 Dec 17 20:44 ./
drwxrwxr-x 209 mbp mbp 12288 Dec 17 20:44 ../
-rw-rw-r--   1 mbp mbp   286 Dec 17 20:44 Cargo.toml
lrwxrwxrwx   1 mbp mbp    11 Dec 17 20:44 LICENSE-GPL -> LICENSE-GPL
-rw-rw-r--   1 mbp mbp    45 Dec 17 20:44 README.md
drwxrwxr-x   2 mbp mbp  4096 Dec 17 20:44 src/

Zed currently treats that as an error reading the symlink, and as a result:

• the link doesn't appear in the tree at all (Broken symlinks not shown in file tree #25181)
• it emits errors to the log

The test about recursive symlinks fails without the fix.

This doesn't completely fix #25181, because the worktree crate also emits entries that it can't canonicalize. I'm not quite sure how to fix that yet, but adding tests and fixing one layer feels like a step forward?

Release Notes:

• N/A