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

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

Index: sdk/lib/_internal/pub/test/get/git/check_out_unfetched_revision_of_cached_repo_test.dart
diff --git a/sdk/lib/_internal/pub/test/get/git/check_out_and_upgrade_test.dart b/sdk/lib/_internal/pub/test/get/git/check_out_unfetched_revision_of_cached_repo_test.dart
similarity index 54%
copy from sdk/lib/_internal/pub/test/get/git/check_out_and_upgrade_test.dart
copy to sdk/lib/_internal/pub/test/get/git/check_out_unfetched_revision_of_cached_repo_test.dart
index f2ff3bfeb545c967d8838a4b01996e88cd1972ab..3ebc04cd0dfa0cbb12a4311ebbd0345fe9002622 100644
--- a/sdk/lib/_internal/pub/test/get/git/check_out_and_upgrade_test.dart
+++ b/sdk/lib/_internal/pub/test/get/git/check_out_unfetched_revision_of_cached_repo_test.dart
@@ -4,16 +4,23 @@
 
 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')
@@ -23,19 +30,11 @@ 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')
@@ -43,8 +42,16 @@ main() {
 
     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, [
       d.dir('git', [
         d.dir('cache', [d.gitPackageRepoCacheDir('foo')]),