detect spurious errors during gitoxide fetch operations specifically

It works by delegating to the respective error types, which know what
to do and whose implementation is modelled after the one found here.

src/cargo/util/network.rs

@@ -79,6 +79,26 @@ fn maybe_spurious(err: &Error) -> bool {
             return true;
         }
     }
+
+    use git::protocol::transport::IsSpuriousError;
+    use git_repository as git;
+
+    if let Some(err) = err.downcast_ref::<git::remote::connect::Error>() {
+        if err.is_spurious() {
+            return true;
+        }
+    }
+    if let Some(err) = err.downcast_ref::<git::remote::fetch::prepare::Error>() {
+        if err.is_spurious() {
+            return true;
+        }
+    }
+    if let Some(err) = err.downcast_ref::<git::remote::fetch::Error>() {
+        if err.is_spurious() {
+            return true;
+        }
+    }
+
     false
 }