utils/github: Add a branch_exists? method

- I wrote this as part of
  https://github.com/issyl0/homebrew-maintenance-utils/blob/main/cmd/find-invalid-head-branches.rb,
  as I needed to check if a branch exists on a repo, and thought I'd properly add
  it in case it's useful to others.

Library/Homebrew/utils/github.rb

@@ -80,6 +80,13 @@ module GitHub
     ["admin", "write"].include?(permission(repo, user)["permission"])
   end
 
+  def branch_exists?(user, repo, branch)
+    API.open_rest("#{API_URL}/repos/#{user}/#{repo}/branches/#{branch}")
+    true
+  rescue API::HTTPNotFoundError
+    false
+  end
+
   def pull_requests(repo, **options)
     url = "#{API_URL}/repos/#{repo}/pulls?#{URI.encode_www_form(options)}"
     API.open_rest(url)