Mute progress animation while Git is running so it doesn't overwrite credential prompts.

BUG=https://code.google.com/p/dart/issues/detail?id=21839
[email protected]

Review URL: https://codereview.chromium.org//845543003

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@42722 260f80e4-7a28-3924-810f-c04153c831b5
diff --git a/lib/src/git.dart b/lib/src/git.dart
index 2c522eb..53d69d8 100644
--- a/lib/src/git.dart
+++ b/lib/src/git.dart
@@ -50,10 +50,14 @@
         "Please ensure Git is correctly installed.");
   }
 
+  log.muteProgress();
   return runProcess(_gitCommand, args, workingDir: workingDir,
       environment: environment).then((result) {
     if (!result.success) throw new GitException(args, result.stderr.join("\n"));
+
     return result.stdout;
+  }).whenComplete(() {
+    log.unmuteProgress();
   });
 }