Add last_update_git_author_name to RepositoryBranch proto (#2406)

Add `last_update_git_author_name` field to contain a author name of the
most recent associated git commit for the branch.
It may be null if the last branch update has no git commit associated.

proto/buf/alpha/registry/v1alpha1/repository_branch.proto

@@ -33,6 +33,9 @@ message RepositoryBranch {
   google.protobuf.Timestamp last_update_time = 5;
   // The ID of the user who updated the branch.
   string last_update_user_id = 6;
+  // The author name of the most recent associated git commit of the branch. May be null
+  // if the last branch update doesn't have a git commit associated.
+  string last_update_git_author_name = 7;
 }
 
 // RepositoryBranchService is the Repository branch service.