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

diff --git a/dev/conductor/core/lib/src/start.dart b/dev/conductor/core/lib/src/start.dart
index 9fb03e7..97b13c3 100644
--- a/dev/conductor/core/lib/src/start.dart
+++ b/dev/conductor/core/lib/src/start.dart
@@ -397,7 +397,10 @@
     ));
 
     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);
diff --git a/dev/conductor/core/test/start_test.dart b/dev/conductor/core/test/start_test.dart
index ecdbfe5..b9e78b7 100644
--- a/dev/conductor/core/test/start_test.dart
+++ b/dev/conductor/core/test/start_test.dart
@@ -277,7 +277,7 @@
           stdout: revision3,
         ),
         const FakeCommand(
-          command: <String>['git', 'merge-base', candidateBranch, 'master'],
+          command: <String>['git', 'merge-base', 'upstream/$candidateBranch', 'upstream/master'],
           stdout: branchPointRevision,
         ),
         // check if commit is tagged, zero exit code means it is tagged
@@ -465,7 +465,7 @@
           stdout: revision3,
         ),
         const FakeCommand(
-          command: <String>['git', 'merge-base', candidateBranch, 'master'],
+          command: <String>['git', 'merge-base', 'upstream/$candidateBranch', 'upstream/master'],
           stdout: branchPointRevision,
         ),
       ];
@@ -639,7 +639,7 @@
           stdout: revision3,
         ),
         const FakeCommand(
-          command: <String>['git', 'merge-base', candidateBranch, 'master'],
+          command: <String>['git', 'merge-base', 'upstream/$candidateBranch', 'upstream/master'],
           stdout: branchPointRevision,
         ),
         // check if commit is tagged, 0 exit code means it is tagged
@@ -826,7 +826,7 @@
           stdout: revision3,
         ),
         const FakeCommand(
-          command: <String>['git', 'merge-base', candidateBranch, 'master'],
+          command: <String>['git', 'merge-base', 'upstream/$candidateBranch', 'upstream/master'],
           stdout: branchPointRevision,
         ),
         // check if commit is tagged, 0 exit code thus it is tagged
@@ -1014,7 +1014,7 @@
           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