[tools] make rev_sdk_deps.dart work with packaged references

third_party/pkg/protobuf (and possibly others) uses a compressed reference file (.git/packed-refs) rather than the .git/refs/heads/(master|main) file expected by rev_sdk_deps.dart.

modify rev_sdk_deps.dart to use "git show-ref --verify refs/remotes/origin/main" instead.

TEST: I ran the script and it produced the expected commit hashes for both a package that uses a "main" default branch and a "master" default branch.

Change-Id: Ibe2ec2cf368d859ca78d1873f287722eb73e481b
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/488262
Reviewed-by: Ryan Macnak <rmacnak@google.com>
Commit-Queue: Brian Quinlan <bquinlan@google.com>

tools/rev_sdk_deps.dart

@@ -201,19 +201,16 @@ class GitHelper {
   }
 
   String get defaultBranchName {
-    var branchNames = Directory(path.join(dir, '.git', 'refs', 'heads'))
-        .listSync()
-        .whereType<File>()
-        .map((f) => path.basename(f.path))
-        .toSet();
-
-    for (var name in ['main', 'master']) {
-      if (branchNames.contains(name)) {
-        return name;
-      }
+    if (Process.runSync(
+          'git',
+          ['show-ref', '--verify', '--quiet', 'refs/remotes/origin/main'],
+          workingDirectory: dir,
+        ).exitCode ==
+        0) {
+      return 'main';
+    } else {
+      return 'master';
     }
-
-    return 'main';
   }
 }