Worktree paths in git panel, take 2 (#26047)

Modified version of #25950. We still use worktree paths, but repo paths
with a status that lie outside the worktree are not excluded; instead,
we relativize them by adding `..`. This makes the list in the git panel
match what you'd get from running `git status` (with the repo's worktree
root as the working directory).

- [x] Implement + test new unrelativization logic
- [x] ~~When collecting repositories, dedup by .git abs path, so
worktrees can share a repo at the project level~~ dedup repos at the
repository selector layer, with repos coming from larger worktrees being
preferred
- [x] Open single-file worktree with diff when activating a path not in
the worktree

Release Notes:

- N/A

crates/util/src/util.rs

@@ -828,6 +828,10 @@ pub fn word_consists_of_emojis(s: &str) -> bool {
     prev_end == s.len()
 }
 
+pub fn default<D: Default>() -> D {
+    Default::default()
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;