[tool] More main-branch detection improvement (#7067)

Follow-up to https://github.com/flutter/plugins/pull/7038 to try to make
it work on LUCI. Looking at the checkout steps of a LUCI run, it looks
like we do a full `fetch` of `origin`, but likely only have a `master`
branch locally. Rather than rely on a specific local branch name
existing, this allows for checking `origin` (and just in case, since
it's another common remote name, `upstream`).

Hopefully fixes https://github.com/flutter/flutter/issues/119330

script/tool/test/common/package_command_test.dart

@@ -875,6 +875,46 @@ packages/b_package/lib/src/foo.dart
           ));
     });
 
+    test(
+        'only tests changed packages relative to the previous commit if '
+        'running on a specific hash from origin/main', () async {
+      processRunner.mockProcessesForExecutable['git-diff'] = <Process>[
+        MockProcess(stdout: 'packages/plugin1/plugin1.dart'),
+      ];
+      processRunner.mockProcessesForExecutable['git-rev-parse'] = <Process>[
+        MockProcess(stdout: 'HEAD'),
+      ];
+      processRunner.mockProcessesForExecutable['git-merge-base'] = <Process>[
+        MockProcess(exitCode: 128), // Fail with a non-1 exit code for 'main'
+        MockProcess(), // Succeed for the variant.
+      ];
+      final RepositoryPackage plugin1 =
+          createFakePlugin('plugin1', packagesDir);
+      createFakePlugin('plugin2', packagesDir);
+
+      final List<String> output = await runCapturingPrint(
+          runner, <String>['sample', '--packages-for-branch']);
+
+      expect(command.plugins, unorderedEquals(<String>[plugin1.path]));
+      expect(
+          output,
+          containsAllInOrder(<Matcher>[
+            contains(
+                '--packages-for-branch: running on a commit from default branch.'),
+            contains(
+                '--packages-for-branch: using parent commit as the diff base'),
+            contains(
+                'Running for all packages that have diffs relative to "HEAD~"'),
+          ]));
+      // Ensure that it's diffing against the prior commit.
+      expect(
+          processRunner.recordedCalls,
+          contains(
+            const ProcessCall(
+                'git-diff', <String>['--name-only', 'HEAD~', 'HEAD'], null),
+          ));
+    });
+
     test(
         'only tests changed packages relative to the previous commit on master',
         () async {