utils/git: remove redundant current_branch

Library/Homebrew/dev-cmd/pr-pull.rb

@@ -385,7 +385,7 @@ module Homebrew
       _, user, repo, pr = *url_match
       odie "Not a GitHub pull request: #{arg}" unless pr
 
-      current_branch = Utils::Git.current_branch(tap.path)
+      current_branch = tap.path.git_branch
       origin_branch = Utils::Git.origin_branch(tap.path).split("/").last
 
       if current_branch != origin_branch || args.branch_okay? || args.clean?

Library/Homebrew/utils/git.rb

@@ -140,7 +140,8 @@ module Utils
     end
 
     def current_branch(repo)
-      Utils.popen_read("git", "-C", repo, "symbolic-ref", "--short", "HEAD").chomp.presence
+      odeprecated "Utils::Git.current_branch(repo)", "Pathname(repo).git_branch"
+      Pathname(repo).extend(GitRepositoryExtension).git_branch
     end
 
     # Special case of `git cherry-pick` that permits non-verbose output and