gcrypt: now supports rsync

Use rsync for gcrypt remotes that are not local to the disk.
(Note that I have punted on supporting http transport for now, it doesn't
seem likely to be very useful.)

This was mostly quite easy, it just uses the rsync special remote to handle
the transfers. The git repository url is converted to a RsyncOptions
structure, which required parsing it separately, since the rsync special
remote only supports rsync urls, which use a different format.

Note that annexed objects are now stored at the top of the gcrypt repo,
rather than inside annex/objects. This simplified the rsync suport,
since it doesn't have to arrange to create that directory. And git-annex
is not going to be run directly within gcrypt repos -- or if in some
strance scenario it was, it would make sense for it to not see the
encrypted objects.

This commit was sponsored by Sheila Miguez

doc/special_remotes/gcrypt.mdwn

@@ -21,6 +21,8 @@ gcrypt:
   for gcrypt to use. This repository should be either empty, or an existing
   gcrypt repositry.
 
+* `shellescape` - See [[rsync]] for the details of this option.
+
 ## notes
 
 For git-annex to store files in a repository on a remote server, you need