Change git-rebase-update to always keep branch 'master'.

git_rebase_update.py

Index: git_rebase_update.py
diff --git a/git_rebase_update.py b/git_rebase_update.py
index a61e53a3ab5b5b94e482e0eef9e71fd82ad9f2c6..5ad0ee176fd2d1296bed485d723f55b961a3ef15 100755
--- a/git_rebase_update.py
+++ b/git_rebase_update.py
@@ -84,6 +84,8 @@ def remove_empty_branches(branch_tree):
   deletions = set()
   downstreams = collections.defaultdict(list)
   for branch, parent in git.topo_iter(branch_tree, top_down=False):
+    if branch == 'master':
+      continue
     downstreams[parent].append(branch)
 
     if git.hash_one(branch) == git.hash_one(parent):