Use main branch as the default branch

R=apolito@google.com

Change-Id: I6a2ed557b9e8d45194b2fe4ea5327ce62020733c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3000134
Reviewed-by: Anthony Polito <apolito@google.com>
Commit-Queue: Josip Sokcevic <sokcevic@google.com>
diff --git a/scm.py b/scm.py
index 812454b..5275d43 100644
--- a/scm.py
+++ b/scm.py
@@ -216,7 +216,7 @@
     except subprocess2.CalledProcessError:
       pass
     # Return default branch
-    return 'refs/remotes/%s/master' % remote
+    return 'refs/remotes/%s/main' % remote
 
   @staticmethod
   def GetBranch(cwd):