Implement changes

Library/Homebrew/diagnostic.rb

@@ -785,6 +785,19 @@ module Homebrew
               git -C "#{coretap_path}" remote set-url origin #{Formatter.url("https://github.com/Homebrew/homebrew-core.git")}
           EOS
         end
+
+        return if ENV["CI"] || ENV["JENKINS_HOME"]
+
+        branch = coretap_path.git_branch
+        return if branch.nil? || branch =~ %r{master}
+
+
+        <<-EOS.undent
+          Homebrew/homebrew-core is not on the master branch
+
+          Check out the master branch by running:
+            git -C "$(brew --repo homebrew/core)" checkout master
+        EOS
       end
 
       def __check_linked_brew(f)

Library/Homebrew/extend/git_repository.rb

@@ -36,6 +36,15 @@ module GitRepositoryExtension
     end
   end
 
+  def git_branch
+    return unless git? && Utils.git_available?
+    cd do
+      Utils.popen_read(
+        "git", "rev-parse", "--abbrev-ref", "HEAD"
+      ).chuzzle
+    end
+  end
+
   def git_last_commit_date
     return unless git? && Utils.git_available?
     cd do

Library/Homebrew/tap.rb

@@ -118,6 +118,12 @@ class Tap
     path.git?
   end
 
+  # git branch for this {Tap}.
+  def git_branch
+    raise TapUnavailableError, name unless installed?
+    path.git_branch
+  end
+
   # git HEAD for this {Tap}.
   def git_head
     raise TapUnavailableError, name unless installed?