improve git repository detection

* configure.in (AC_CONFIG_FILES): $srcdir/.git can be a file pointing
  the real git_dir, such as when the git working tree is a "linked
  working tree" (a working tree created by git-worktree). So use
  git-rev-parse --git-dir to check if $srcdir is the top-level of a git
  repository, not just checking if the $srcdir/.git directory does exist
  or not.  [ruby-core:74759] [Bug #12239]
* tool/change_maker.rb: use tool/vcs.rb to detect VCS. This used to have
  its own VCS detection code, while we have tool/vcs.rb.
* tool/vcs.rb (detect): remove code duplication

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@54468 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

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