add-grammar: use abspath when caching license file

Fixes breakage caused by passing a relative dependency path to licensed.

script/add-grammar

@@ -111,6 +111,7 @@ log "Confirming license"
 if repo_old
   command('script/licensed')
 else
+  repo_new = File.absolute_path(repo_new)
   command('script/licensed', '--module', repo_new)
 end