fix: When removing all shallow commits from shallow file, delete it.

Previously it would leave an empty file, which will be ignored by the implementation
but might be confusing to users.

gix/src/shallow.rs

@@ -36,6 +36,11 @@ pub mod write {
                     ShallowUpdate::Unshallow(id) => shallow_commits.retain(|oid| oid != id),
                 }
             }
+            if shallow_commits.is_empty() {
+                std::fs::remove_file(file.resource_path())?;
+                drop(file);
+                return Ok(());
+            }
 
             if shallow_commits.is_empty() {
                 if let Err(err) = std::fs::remove_file(file.resource_path()) {

gix/tests/clone/mod.rs

@@ -72,6 +72,10 @@ mod blocking_io {
             !repo.is_shallow(),
             "both methods agree - if there are no shallow commits, it shouldn't think the repo is shallow"
         );
+        assert!(
+            !repo.shallow_file().exists(),
+            "when the repo is not shallow anymore, there is no need for a shallow file"
+        );
         assert!(
             repo.head_id()?.ancestors().all()?.count() > shallow_commit_count,
             "there are more commits now as the history is complete"