feat(test.rs): Lookup the commit with cat-file to avoid re-fetches

build_system/src/test.rs

@@ -511,6 +511,18 @@ fn setup_rustc(env: &mut Env, args: &TestArg) -> Result<PathBuf, String> {
         Some(commit_hash) => commit_hash,
         None => return Err("Couldn't retrieve rustc commit hash".to_string()),
     };
+    let has_commit = {
+        if let Ok(ty) = run_command_with_env(
+            &[&"git", &"cat-file", &"-t", &rustc_commit.as_str()],
+            rust_dir,
+            Some(env),
+        ) {
+            String::from_utf8_lossy(&ty.stdout).to_string() == "commit"
+        } else {
+            false
+        }
+    };
+    if !has_commit {
         run_command_with_output_and_env(
             &[
                 &"git",
@@ -521,7 +533,8 @@ fn setup_rustc(env: &mut Env, args: &TestArg) -> Result<PathBuf, String> {
             ],
             rust_dir,
             Some(env),
-    )?;
+        )?
+    };
     if rustc_commit != "unknown" {
         run_command_with_output_and_env(
             &[&"git", &"checkout", &rustc_commit],