Fix issue for MinGW / MSYS2 builds and testing

See ruby/ruby#1553 & #1876

Fixes test-all isssues MinGW / Windows

Author: MSP-Greg

lib/rubygems/test_case.rb

@@ -484,7 +484,7 @@ def git_gem name = 'a', version = 1
 
       system @git, 'add', gemspec
       system @git, 'commit', '-a', '-m', 'a non-empty commit message', '--quiet'
-      head = Gem::Util.popen('git', 'rev-parse', 'master').strip
+      head = Gem::Util.popen(@git, 'rev-parse', 'master').strip
     end
 
     return name, git_spec.version, directory, head