Add GetRemoteMasterHash() function to git_utils.py.

slave/skia_slave_scripts/utils/git_utils.py

Index: slave/skia_slave_scripts/utils/git_utils.py
diff --git a/slave/skia_slave_scripts/utils/git_utils.py b/slave/skia_slave_scripts/utils/git_utils.py
index 0fa688daf3601481052e61eaeded752a7f6d2645..da7f0976ef2dc07852fd506a11d5bc9c9d6085c7 100644
--- a/slave/skia_slave_scripts/utils/git_utils.py
+++ b/slave/skia_slave_scripts/utils/git_utils.py
@@ -40,3 +40,7 @@ def MergeAbort():
 def ShortHash(commit):
   """Return short hash of the specified commit."""
   return shell_utils.run([GIT, 'show', commit, '--format=%h', '-s']).rstrip()
+
+def GetRemoteMasterHash(git_url):
+  return shell_utils.run([GIT, 'ls-remote', git_url, '--verify',
+                          'refs/heads/master'])