Enable clippy::single_char_pattern (#8727)

This PR enables the
[`clippy::single_char_pattern`](https://rust-lang.github.io/rust-clippy/master/index.html#/single_char_pattern)
rule and fixes the outstanding violations.

Release Notes:

- N/A

crates/project_panel/src/project_panel.rs

@@ -607,7 +607,7 @@ impl ProjectPanel {
                 worktree_id,
                 entry_id: NEW_ENTRY_ID,
             });
-            let new_path = entry.path.join(&filename.trim_start_matches("/"));
+            let new_path = entry.path.join(&filename.trim_start_matches('/'));
             if path_already_exists(new_path.as_path()) {
                 return None;
             }