Add tag write access to merge script.

tools/push-to-trunk/common_includes.py

Index: tools/push-to-trunk/common_includes.py
diff --git a/tools/push-to-trunk/common_includes.py b/tools/push-to-trunk/common_includes.py
index 7ea39f73c95e80c86e10eb960241dc3e7683b76d..1d434f1f339568af41150040be2f8a88a0f8ad0b 100644
--- a/tools/push-to-trunk/common_includes.py
+++ b/tools/push-to-trunk/common_includes.py
@@ -346,7 +346,7 @@ class GitSvnInterface(VCInterface):
     self.step.GitSVNTag(tag)
 
 
-class GitReadOnlyMixin(VCInterface):
+class GitTagsOnlyMixin(VCInterface):
   def Pull(self):
     self.step.GitPull()
 
@@ -375,8 +375,12 @@ class GitReadOnlyMixin(VCInterface):
       return "origin/%s" % name
     return "origin/branch-heads/%s" % name
 
+  def Tag(self, tag):
+    self.step.Git("tag %s" % tag)
+    self.step.Git("push origin %s" % tag)
+
 
-class GitReadSvnWriteInterface(GitReadOnlyMixin, GitSvnInterface):
+class GitReadSvnWriteInterface(GitTagsOnlyMixin, GitSvnInterface):
   pass