vcs.rb: do not use -C for older git
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@49705 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
vcs.rb: do not use -C for older git
of -C option which is not supported by older git.
[ruby-dev:48880] [Bug #10890]
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@49705 b2dd03c8-39d4-4d8f-98ff-823fe69b080e