[ruby-cvs:78306] d7f100226d (master): Skip test_validate_gemspec when tarball and git installed too

Kazuhiro NISHIYAMA	2019-11-23 01:29:53 +0900 (Sat, 23 Nov 2019)

  New Revision: d7f100226d

  https://github.com/ruby/ruby/commit/d7f100226d

  Log:
    Skip test_validate_gemspec when tarball and git installed too
    
    `git --version` failed as expected when git is not installed,
    but unexpectedly pass when git installed and pwd is not in git working directory.
    So use `git rev-parse` instead, and it failed when git installed too.

  Modified files:
    test/ruby/test_default_gems.rb