Don't run "git fetch" multiple times for the same repository.

sdk/lib/_internal/pub/lib/src/source/git.dart

Index: sdk/lib/_internal/pub/lib/src/source/git.dart
diff --git a/sdk/lib/_internal/pub/lib/src/source/git.dart b/sdk/lib/_internal/pub/lib/src/source/git.dart
index afec7bf0a70e15388184b967b7ae6c3611c9640a..09a6a44ffcc75f51d01fd7878558ef6c8cad0a49 100644
--- a/sdk/lib/_internal/pub/lib/src/source/git.dart
+++ b/sdk/lib/_internal/pub/lib/src/source/git.dart
@@ -20,6 +20,10 @@ class GitSource extends Source {
 
   final bool shouldCache = true;
 
+  /// The paths to the canonical clones of repositories for which "git fetch"
+  /// has already been run during this run of pub.
+  final _updatedRepos = new Set<String>();
+
   GitSource();
 
   /// Clones a Git repo to the local filesystem.
@@ -151,8 +155,13 @@ class GitSource extends Source {
   /// [id].
   ///
   /// This assumes that the canonical clone already exists.
-  Future _updateRepoCache(PackageId id) =>
-      git.run(["fetch"], workingDir: _repoCachePath(id));
+  Future _updateRepoCache(PackageId id) {
+    var path = _repoCachePath(id);
+    if (_updatedRepos.contains(path)) return new Future.value();
+    return git.run(["fetch"], workingDir: path).then((_) {
+      _updatedRepos.add(path);
+    });
+  }
 
   /// Runs "git rev-parse" in the canonical clone of the repository referred to
   /// by [id] on the effective ref of [id].