add Backend.GitRemoteAnnex

Making GITBUNDLE be in the backend list allows those keys to be
hashed to verify, both when git-remote-annex downloads them, and by other
transfers and by git fsck.

GITMANIFEST is not in the backend list, because those keys will never be
stored in .git/annex/objects and can't be verified in any case.

This does mean that git-annex version will include GITBUNDLE in the list
of backends.

Also documented these in backends.mdwn

Sponsored-by: Kevin Mueller on Patreon

git-annex.cabal

@@ -580,6 +580,7 @@ Executable git-annex
     Author
     Backend
     Backend.External
+    Backend.GitRemoteAnnex
     Backend.Hash
     Backend.URL
     Backend.Utilities