Quote git_dir when used in git command execution

This prevents exceptions (and possibly worse) when attempting to execute git against repositories whose path contains a space.
Note that this doesn't try to protect against stranger cases, like directory names that contain double-quotes, but I'm not familiar with all the special characters that would need to be escaped here.

Author: bklang

lib/code_climate/test_reporter/git.rb

@@ -45,7 +45,7 @@ def branch_from_git
         end
 
         def git(command)
-          `git --git-dir=#{git_dir}/.git #{command}`
+          `git --git-dir="#{git_dir}"/.git #{command}`
         end
 
         def git_dir