Properly detect if git revisions aren't in a git repository.

sdk/lib/_internal/pub_generated/test/get/git/check_out_unfetched_revision_of_cached_repo_test.dart

Index: sdk/lib/_internal/pub_generated/test/get/git/check_out_unfetched_revision_of_cached_repo_test.dart
diff --git a/sdk/lib/_internal/pub_generated/test/get/git/check_out_and_upgrade_test.dart b/sdk/lib/_internal/pub_generated/test/get/git/check_out_unfetched_revision_of_cached_repo_test.dart
similarity index 55%
copy from sdk/lib/_internal/pub_generated/test/get/git/check_out_and_upgrade_test.dart
copy to sdk/lib/_internal/pub_generated/test/get/git/check_out_unfetched_revision_of_cached_repo_test.dart
index 99366642bac835a7ea66c896beca5681eea36a59..277f2d189a60934f6ddb077085ed2314dce494be 100644
--- a/sdk/lib/_internal/pub_generated/test/get/git/check_out_and_upgrade_test.dart
+++ b/sdk/lib/_internal/pub_generated/test/get/git/check_out_unfetched_revision_of_cached_repo_test.dart
@@ -4,16 +4,24 @@
 
 library pub_tests;
 
+import 'package:path/path.dart' as p;
 import 'package:scheduled_test/scheduled_test.dart';
 
+import '../../../lib/src/io.dart';
 import '../../descriptor.dart' as d;
 import '../../test_pub.dart';
 
 main() {
   initConfig();
-  integration('checks out and upgrades a package from Git', () {
+  // Regression test for issue 20947.
+  integration(
+      'checks out an unfetched and locked revision of a cached ' 'repository',
+      () {
     ensureGit();
 
+    // In order to get a lockfile that refers to a newer revision than is in the
+    // cache, we'll switch between two caches. First we ensure that the repo is
+    // in the first cache.
     d.git('foo.git', [d.libDir('foo'), d.libPubspec('foo', '1.0.0')]).create();
 
     d.appDir({
@@ -24,27 +32,28 @@ main() {
 
     pubGet();
 
-    d.dir(
-        cachePath,
-        [
-            d.dir(
-                'git',
-                [
-                    d.dir('cache', [d.gitPackageRepoCacheDir('foo')]),
-                    d.gitPackageRevisionCacheDir('foo')])]).validate();
-
-    d.dir(
-        packagesPath,
-        [d.dir('foo', [d.file('foo.dart', 'main() => "foo";')])]).validate();
+    // Switch to a new cache.
+    schedule(
+        () =>
+            renameDir(p.join(sandboxDir, cachePath), p.join(sandboxDir, "$cachePath.old")));
 
+    // Make the lockfile point to a new revision of the git repository.
     d.git(
         'foo.git',
         [d.libDir('foo', 'foo 2'), d.libPubspec('foo', '1.0.0')]).commit();
 
     pubUpgrade(output: contains("Changed 1 dependency!"));
 
-    // When we download a new version of the git package, we should re-use the
-    // git/cache directory but create a new git/ directory.
+    // Switch back to the old cache.
+    schedule(() {
+      var cacheDir = p.join(sandboxDir, cachePath);
+      deleteEntry(cacheDir);
+      renameDir(p.join(sandboxDir, "$cachePath.old"), cacheDir);
+    });
+
+    // Get the updated version of the git dependency based on the lockfile.
+    pubGet();
+
     d.dir(
         cachePath,
         [