Print out the working directory when git fails.

utils/pub/git.dart

Index: utils/pub/git.dart
diff --git a/utils/pub/git.dart b/utils/pub/git.dart
index 31e8b27c9bc20060d55741880fd7a882cb2bce56..5bc5cd152b267e4530e8f8465dfef046a64aabc5 100644
--- a/utils/pub/git.dart
+++ b/utils/pub/git.dart
@@ -29,6 +29,7 @@ Future<List<String>> run(List<String> args, [String workingDir]) {
   }).transform((result) {
     if (!result.success) throw new Exception(
         'Git error. Command: git ${Strings.join(args, " ")}\n'
+        'Working directory: $currentWorkingDir\n'
         '${Strings.join(result.stderr, "\n")}');
 
     return result.stdout;