Limit uplevel travarsing

* tool/vcs.rb (VCS.detect): limit level of travarsing parent
  directories, 0 by the default.  curretly always detecting at the
  source directory itself.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@66305 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

tool/vcs.rb

@@ -124,12 +124,16 @@ class VCS
     @@dirs << [dir, self, pred]
   end
 
-  def self.detect(path)
+  def self.detect(path, uplevel_limit: 0)
     curr = path
     begin
       @@dirs.each do |dir, klass, pred|
         return klass.new(curr) if pred ? pred[curr, dir] : File.directory?(File.join(curr, dir))
       end
+      if uplevel_limit
+        break if uplevel_limit.zero?
+        uplevel_limit -= 1
+      end
       prev, curr = curr, File.realpath(File.join(curr, '..'))
     end until curr == prev # stop at the root directory
     raise VCS::NotFoundError, "does not seem to be under a vcs: #{path}"