[flutter_conductor] Fix conductor start branch point (#100170)

dev/conductor/core/lib/src/start.dart

@@ -397,7 +397,10 @@ class StartContext extends Context {
     ));
 
     final String frameworkHead = await framework.reverseParse('HEAD');
-    final String branchPoint = await framework.branchPoint(candidateBranch, FrameworkRepository.defaultBranch);
+    final String branchPoint = await framework.branchPoint(
+      '${framework.upstreamRemote.name}/$candidateBranch',
+      '${framework.upstreamRemote.name}/${FrameworkRepository.defaultBranch}',
+    );
     final bool atBranchPoint = branchPoint == frameworkHead;
 
     final ReleaseType releaseType = computeReleaseType(lastVersion, atBranchPoint);

dev/conductor/core/test/start_test.dart

@@ -277,7 +277,7 @@ void main() {
           stdout: revision3,
         ),
         const FakeCommand(
-          command: <String>['git', 'merge-base', candidateBranch, 'master'],
+          command: <String>['git', 'merge-base', 'upstream/$candidateBranch', 'upstream/master'],
           stdout: branchPointRevision,
         ),
         // check if commit is tagged, zero exit code means it is tagged
@@ -465,7 +465,7 @@ void main() {
           stdout: revision3,
         ),
         const FakeCommand(
-          command: <String>['git', 'merge-base', candidateBranch, 'master'],
+          command: <String>['git', 'merge-base', 'upstream/$candidateBranch', 'upstream/master'],
           stdout: branchPointRevision,
         ),
       ];
@@ -639,7 +639,7 @@ void main() {
           stdout: revision3,
         ),
         const FakeCommand(
-          command: <String>['git', 'merge-base', candidateBranch, 'master'],
+          command: <String>['git', 'merge-base', 'upstream/$candidateBranch', 'upstream/master'],
           stdout: branchPointRevision,
         ),
         // check if commit is tagged, 0 exit code means it is tagged
@@ -826,7 +826,7 @@ void main() {
           stdout: revision3,
         ),
         const FakeCommand(
-          command: <String>['git', 'merge-base', candidateBranch, 'master'],
+          command: <String>['git', 'merge-base', 'upstream/$candidateBranch', 'upstream/master'],
           stdout: branchPointRevision,
         ),
         // check if commit is tagged, 0 exit code thus it is tagged
@@ -1014,7 +1014,7 @@ void main() {
           stdout: branchPointRevision,
         ),
         const FakeCommand(
-          command: <String>['git', 'merge-base', candidateBranch, 'master'],
+          command: <String>['git', 'merge-base', 'upstream/$candidateBranch', 'upstream/master'],
           stdout: branchPointRevision,
         ),
         // check if commit is tagged