[tool] Remove use of FETCH_HEAD (#10357)

Changes the default branch used to find merge bases from `FETCH_HEAD` to `main`. We were arranging for it to be `main` in CI, and locally it was an unpredictable value, so just hard-coding to `main` is likely to give better behavior locally with no change to CI behavior.

Fixes flutter/flutter#176738

Author: stuartmorgan-g

.ci/scripts/prepare_tool.sh

@@ -4,8 +4,9 @@
 # found in the LICENSE file.
 set -e
 
-# To set FETCH_HEAD for "git merge-base" to work
+# Ensure that 'main' is present for diffing.
 git fetch origin main
+git branch main origin/main
 
 cd script/tool
 dart pub get

script/tool/lib/src/common/git_version_finder.dart

@@ -15,7 +15,7 @@ class GitVersionFinder {
       : assert(baseSha == null || baseBranch == null,
             'At most one of baseSha and baseBranch can be provided'),
         _baseSha = baseSha,
-        _baseBranch = baseBranch ?? 'FETCH_HEAD';
+        _baseBranch = baseBranch ?? 'main';
 
   /// The top level directory of the git repo.
   ///

script/tool/lib/src/common/package_command.dart

@@ -129,7 +129,7 @@ abstract class PackageCommand extends Command<void> {
     argParser.addOption(_baseBranchArg,
         help: 'The base branch whose merge base is used as the base SHA if '
             '--$_baseShaArg is not provided. \n'
-            'If not specified, FETCH_HEAD is used as the base branch.');
+            'If not specified, "main" is used as the base branch.');
     argParser.addFlag(_logTimingArg,
         help: 'Logs timing information.\n\n'
             'Currently only logs per-package timing for multi-package commands, '

script/tool/test/common/git_version_finder_test.dart

@@ -66,7 +66,7 @@ file2/file2.cc
     final GitVersionFinder finder = GitVersionFinder(gitDir);
     await finder.getChangedFiles();
     verify(gitDir.runCommand(
-        <String>['merge-base', '--fork-point', 'FETCH_HEAD', 'HEAD'],
+        <String>['merge-base', '--fork-point', 'main', 'HEAD'],
         throwOnError: false));
     verify(gitDir.runCommand(
         <String>['diff', '--name-only', mergeBaseResponse, 'HEAD']));

script/tool/test/version_check_command_test.dart

@@ -150,7 +150,7 @@ void main() {
           gitProcessRunner.recordedCalls,
           containsAllInOrder(const <ProcessCall>[
             ProcessCall('git-merge-base',
-                <String>['--fork-point', 'FETCH_HEAD', 'HEAD'], null),
+                <String>['--fork-point', 'main', 'HEAD'], null),
             ProcessCall('git-show',
                 <String>['abc123:packages/plugin/pubspec.yaml'], null),
           ]));