Allow 'main' as a local main branch name.

Avoiding 'master' as a local branch name per naming guidelines hits this
special case and won't allow upload of child branches. Even though more
comprehensive efforts (crbug.com/1095735) have yet to decide on an
official preferred main branch name this change is small and unlikely
to break things (I hope).

This supports people using a "git pull" "merge" workflow.

Change-Id: I8aca044aeb4178c6671e3a1590617f666505b42e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2377189
Reviewed-by: Josip Sokcevic <sokcevic@google.com>
Commit-Queue: Josip Sokcevic <sokcevic@google.com>
Auto-Submit: Glen Robertson <glenrob@chromium.org>

git_cl.py

@@ -2360,6 +2360,8 @@ class Changelist(object):
 
     if upstream_branch_name == 'master':
       return self.GetCommonAncestorWithUpstream()
+    if upstream_branch_name == 'main':
+      return self.GetCommonAncestorWithUpstream()
 
     # Check the squashed hash of the parent.
     # TODO(tandrii): consider checking parent change in Gerrit and using its