https://git.ruby-lang.org/ruby.git/commit/?id=025206d0dd

From 025206d0dd29266771f166eb4f59609af602213a Mon Sep 17 00:00:00 2001
From: Nobuyoshi Nakada <nobu@r...>
Date: Thu, 9 May 2019 14:11:43 +0900
Subject: Fallback to an invalid branch name if no branch found


diff --git a/tool/vcs.rb b/tool/vcs.rb
index 1d8fe90..be7b672 100644
--- a/tool/vcs.rb
+++ b/tool/vcs.rb
@@ -403,6 +403,7 @@ class VCS https://github.com/ruby/ruby/blob/trunk/tool/vcs.rb#L403
         (branch = branch_list[0]).strip! unless branch_list.empty?
       end
       branch.chomp!
+      branch = ":detached:" if branch.empty?
       upstream = cmd_read_at(srcdir, [gitcmd + %W[branch --list --format=%(upstream:short) #{branch}]])
       upstream.chomp!
       title = cmd_read_at(srcdir, [gitcmd + %W[log --format=%s -n1 #{upstream}..HEAD]])
-- 
cgit v0.10.2


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