Fix GitRepo specs for git < 1.8.5

`git` < 1.8.5 does not update the `.gitmodules` file with submodule
changes, so when we are looking at the current state of the work tree
we need to check if the submodule actually exists via another method,
since the `.gitmodules` file we parsed does not represent reality.

At some point we're likely going to want to have a helper that provides
a convenient way for us to conditionally run code based on the `git`
version Overcommit is using.

Author: sds

lib/overcommit/git_repo.rb

@@ -229,6 +229,17 @@ def submodules(options = {})
 
       modules = []
       IniParse.parse(`git show #{ref}:.gitmodules`).each do |section|
+        # git < 1.8.5 does not update the .gitmodules file with submodule
+        # changes, so when we are looking at the current state of the work tree,
+        # we need to check if the submodule actually exists via another method,
+        # since the .gitmodules file we parsed does not represent reality.
+        unless ref
+          result = Overcommit::Utils.execute(%W[
+            git submodule status #{section['path']}
+          ])
+          next unless result.success?
+        end
+
         modules << Submodule.new(section['path'], section['url'])
       end