Default to origin's default branch instead of always origin/master.

BUG=1023031

Change-Id: I4bf3e33932af40600646f070f057a7c8c0661f33
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1954624
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>
Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>
Auto-Submit: Jeffrey Yasskin <jyasskin@chromium.org>
diff --git a/git_common.py b/git_common.py
index 9feb04b..0fa07e8 100644
--- a/git_common.py
+++ b/git_common.py
@@ -665,8 +665,16 @@
   return run('rev-parse', '--show-toplevel')
 
 
+def upstream_default():
+  """Returns the default branch name of the origin repository."""
+  try:
+    return run('rev-parse', '--abbrev-ref', 'origin/HEAD')
+  except subprocess2.CalledProcessError:
+    return 'origin/master'
+
+
 def root():
-  return get_config('depot-tools.upstream', 'origin/master')
+  return get_config('depot-tools.upstream', upstream_default())
 
 
 @contextlib.contextmanager