test_gem_source_git.rb: drop intended error message

* test/rubygems/test_gem_source_git.rb (test_rev_parse): dispose
  error message by git, which should be intended.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47787 b2dd03c8-39d4-4d8f-98ff-823fe69b080e