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/worktree/src/worktree.rs

@@ -58,7 +58,7 @@ use std::{
     future::Future,
     mem::{self},
     ops::{Deref, DerefMut},
-    path::{Path, PathBuf},
+    path::{Component, Path, PathBuf},
     pin::Pin,
     sync::{
         atomic::{self, AtomicU32, AtomicUsize, Ordering::SeqCst},
@@ -212,7 +212,11 @@ impl RepositoryEntry {
         self.work_directory.relativize(path)
     }
 
-    pub fn unrelativize(&self, path: &RepoPath) -> Option<Arc<Path>> {
+    pub fn try_unrelativize(&self, path: &RepoPath) -> Option<Arc<Path>> {
+        self.work_directory.try_unrelativize(path)
+    }
+
+    pub fn unrelativize(&self, path: &RepoPath) -> Arc<Path> {
         self.work_directory.unrelativize(path)
     }
 
@@ -491,7 +495,7 @@ impl WorkDirectory {
     }
 
     /// This is the opposite operation to `relativize` above
-    pub fn unrelativize(&self, path: &RepoPath) -> Option<Arc<Path>> {
+    pub fn try_unrelativize(&self, path: &RepoPath) -> Option<Arc<Path>> {
         match self {
             WorkDirectory::InProject { relative_path } => Some(relative_path.join(path).into()),
             WorkDirectory::AboveProject {
@@ -504,6 +508,33 @@ impl WorkDirectory {
         }
     }
 
+    pub fn unrelativize(&self, path: &RepoPath) -> Arc<Path> {
+        match self {
+            WorkDirectory::InProject { relative_path } => relative_path.join(path).into(),
+            WorkDirectory::AboveProject {
+                location_in_repo, ..
+            } => {
+                if &path.0 == location_in_repo {
+                    // Single-file worktree
+                    return location_in_repo
+                        .file_name()
+                        .map(Path::new)
+                        .unwrap_or(Path::new(""))
+                        .into();
+                }
+                let mut location_in_repo = &**location_in_repo;
+                let mut parents = PathBuf::new();
+                loop {
+                    if let Ok(segment) = path.strip_prefix(location_in_repo) {
+                        return parents.join(segment).into();
+                    }
+                    location_in_repo = location_in_repo.parent().unwrap_or(Path::new(""));
+                    parents.push(Component::ParentDir);
+                }
+            }
+        }
+    }
+
     pub fn display_name(&self) -> String {
         match self {
             WorkDirectory::InProject { relative_path } => relative_path.display().to_string(),
@@ -1422,6 +1453,19 @@ impl Worktree {
             worktree_scan_id: scan_id as u64,
         })
     }
+
+    pub fn dot_git_abs_path(&self, work_directory: &WorkDirectory) -> PathBuf {
+        let mut path = match work_directory {
+            WorkDirectory::InProject { relative_path } => self.abs_path().join(relative_path),
+            WorkDirectory::AboveProject { absolute_path, .. } => absolute_path.as_ref().to_owned(),
+        };
+        path.push(".git");
+        path
+    }
+
+    pub fn is_single_file(&self) -> bool {
+        self.root_dir().is_none()
+    }
 }
 
 impl LocalWorktree {
@@ -5509,7 +5553,7 @@ impl BackgroundScanner {
 
                 let mut new_entries_by_path = SumTree::new(&());
                 for (repo_path, status) in statuses.entries.iter() {
-                    let project_path = repository.work_directory.unrelativize(repo_path);
+                    let project_path = repository.work_directory.try_unrelativize(repo_path);
 
                     new_entries_by_path.insert_or_replace(
                         StatusEntry {