Fix git_dir output on Git < 2.5

Strangely enough the --common-git-dir option just returns
"--git-common-dir" with git version 1.17.10 if no $GIT_COMMON_DIR
defined. The only way we can get correct answer is to stick to
--git-dir. As long we are running Git 2.5+ we should use
--git-common-dir, otherwise we can use --git-dir.

Author: Antronin

lib/overcommit/utils.rb

@@ -59,7 +59,9 @@ def repo_root
       def git_dir
         @git_dir ||=
           begin
-            result = execute(%w[git rev-parse --git-common-dir])
+            cmd = %w[git rev-parse]
+            cmd << (GIT_VERSION < '2.5' ? '--git-dir' : '--git-common-dir')
+            result = execute(cmd)
             unless result.success?
               raise Overcommit::Exceptions::InvalidGitRepo,
                     'Unable to determine location of GIT_DIR. ' \