make-snapshot: default to the toplevel directory

As this tool has been intended to use in a working directory,
assume that the toplevel directory is under the VCS, and SVN will
no longer be canonical.

tool/make-snapshot

@@ -569,8 +569,12 @@ elsif $svn
 elsif $git
   abort "#{File.basename $0}: use -srcdir with cloned local repository"
 else
+  begin
+    vcs = VCS.detect(File.expand_path("../..", __FILE__))
+  rescue VCS::NotFoundError
     vcs = VCS::SVN.new(SVNURL)
   end
+end
 
 success = true
 revisions.collect {|rev| package(vcs, rev, destdir, tmp)}.flatten.each do |name|