Fix more bugs in files (#16241)

Fixes:
- [x] an issue where directories would only match by prefix, causing
both a directory and a file to be matched if in the same directory
- [x] An issue where you could not continue a file completion when
selecting a directory, as `tab` on a file would always run the command.
This effectively disabled directory sub queries.
- [x] Inconsistent rendering of files and directories in the slash
command

Release Notes:

- N/A

---------

Co-authored-by: max <max@zed.dev>

crates/assistant/Cargo.toml

@@ -39,6 +39,7 @@ feature_flags.workspace = true
 fs.workspace = true
 futures.workspace = true
 fuzzy.workspace = true
+globset.workspace = true
 gpui.workspace = true
 handlebars.workspace = true
 heed.workspace = true