Add comment linking to issue for future reference

Author: ericholscher

readthedocs/vcs_support/backends/git.py

@@ -148,6 +148,8 @@ def use_shallow_clone(self):
         return not self.project.has_feature(Feature.DONT_SHALLOW_CLONE)
 
     def fetch(self):
+        # --force lets us checkout branches that are not fast-forwarded
+        # https://github.com/readthedocs/readthedocs.org/issues/6097
         cmd = ['git', 'fetch', 'origin',
                '--force', '--tags', '--prune', '--prune-tags']