Fix custom hooksPath test on Windows

This was breaking because Windows includes the drive letter in absolute
paths:

    1) Overcommit::GitConfig.hooks_path when explicitly set to an
       absolute path returns the absolute path
       Failure/Error: expect(subject).to eq '/etc/hooks'

         expected: "/etc/hooks"
              got: "C:/etc/hooks"

Author: sds

spec/overcommit/git_config_spec.rb

@@ -60,7 +60,7 @@
       end
 
       it 'returns the absolute path' do
-        expect(subject).to eq '/etc/hooks'
+        expect(subject).to eq File.absolute_path('/etc/hooks')
       end
     end