vcs.rb: fake string for branch names

* tool/vcs.rb (VCS::GIT#branch): make fake string for branch names
  to suppress expanding as a path.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@49290 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

tool/vcs.rb

@@ -208,8 +208,10 @@ class VCS
       [last, changed, modified]
     end
 
+    Branch = Struct.new(:to_str)
+
     def branch(name)
-      name
+      Branch.new(name)
     end
 
     alias tag branch