vcs.rb: srcdir parameter

* tool/vcs.rb (VCS#get_revisions): add srcdir optional parameter
  to SVN.get_revisions and GIT.get_revisions, instead of change
  working directory.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48834 b2dd03c8-39d4-4d8f-98ff-823fe69b080e