inheritable annex.securehashesonly

inheritable annex.securehashesonly

* init: When annex.securehashesonly has been set with git-annex config,
  copy that value to the annex.securehashesonly git config.
* config --set: As well as setting value in git-annex branch,
  set local gitconfig. This is needed especially for
  annex.securehashesonly, which is read only from local gitconfig and not
  the git-annex branch.

doc/todo/sha1_collision_embedding_in_git-annex_keys.mdwn has the
rationalle for doing it this way. There's no perfect solution; this
seems to be the least-bad one.

This commit was supported by the NSF-funded DataLad project.