GIT_SUBMODULE fix: File not found .git/HEAD error (#168)

* Fix File not found .git/HEAD error
* Verify if git path output is absolute

Author: Bjoe

cmake/modules/hunter_pack_git_submodule.cmake

@@ -139,6 +139,9 @@ function(hunter_pack_git_submodule)
   endif()
 
   set(head_file "${output}")
+  if(NOT IS_ABSOLUTE "${head_file}")
+    set(head_file "${submodule_dir}/${head_file}")
+  endif()
   if(NOT EXISTS "${head_file}")
     hunter_internal_error("File not found: '${head_file}'")
   endif()
@@ -182,6 +185,9 @@ function(hunter_pack_git_submodule)
   endif()
 
   set(ref_file "${output}")
+  if(NOT IS_ABSOLUTE "${ref_file}")
+    set(ref_file "${submodule_dir}/${ref_file}")
+  endif()
   if(NOT EXISTS "${ref_file}")
     hunter_internal_error("File not found: ${ref_file}")
   endif()