setup desktop: Get the remote's default branch

Instead of guessing

lib/setup/clone.dart

@@ -50,6 +50,17 @@ Future<Result<void>> cloneRemote({
     if (r.isFailure) {
       return fail(r);
     }
+
+    var branchR = await gitDefaultBranchViaExecutable(
+      repoPath: repoPath,
+      privateKey: sshPrivateKey,
+      privateKeyPassword: sshPassword,
+      remoteName: remoteName,
+    );
+    if (r.isFailure) {
+      return fail(r);
+    }
+    remoteBranchName = branchR.getOrThrow();
   }
   Log.i("Using remote branch: $remoteBranchName");
 

lib/utils/git_desktop.dart

@@ -1,6 +1,7 @@
 // GIT_SSH_COMMAND='ssh -i private_key_file -o IdentitiesOnly=yes' git clone user@host:repo.git
 
-import 'dart:io' show Directory, File, Process, ProcessStartMode;
+import 'dart:convert';
+import 'dart:io';
 
 import 'package:dart_git/utils/result.dart';
 
@@ -72,3 +73,57 @@ Future<Result<void>> _gitCommandViaExecutable({
 
   return Result(null);
 }
+
+// Default branch - git remote show origin | grep 'HEAD branch'
+Future<Result<String>> gitDefaultBranchViaExecutable({
+  required String repoPath,
+  required String privateKey,
+  required String privateKeyPassword,
+  required String remoteName,
+}) async {
+  assert(repoPath.startsWith('/'));
+  if (privateKeyPassword.isNotEmpty) {
+    var ex = Exception("SSH Keys with passwords are not supported");
+    return Result.fail(ex);
+  }
+
+  var dir = Directory.systemTemp.createTempSync();
+  var temp = File("${dir.path}/key");
+  temp.writeAsString(privateKey);
+
+  var process = await Process.start(
+    'git',
+    [
+      'remote',
+      'show',
+      remoteName,
+    ],
+    workingDirectory: repoPath,
+    environment: {
+      'GIT_SSH_COMMAND': 'ssh -i ${temp.path} -o IdentitiesOnly=yes',
+    },
+  );
+
+  var exitCode = await process.exitCode;
+  await dir.delete(recursive: true);
+
+  if (exitCode != 0) {
+    var ex = Exception("Failed to fetch, exitCode: $exitCode");
+    return Result.fail(ex);
+  }
+
+  var stdoutB = <int>[];
+  await for (var d in process.stdout) {
+    stdoutB.addAll(d);
+  }
+  var stdout = utf8.decode(stdoutB);
+  for (var line in LineSplitter.split(stdout)) {
+    if (line.contains('HEAD branch:')) {
+      var branch = line.split(':')[1].trim();
+      return Result(branch);
+    }
+  }
+
+  var ex = Exception('Default Branch not found');
+  return Result.fail(ex);
+}