add remote.<name>.gcrypt-signingkey config

This is needed by git-annex assistant when it sets up a gcrypt repository,
to ensure that the gpg key it was asked to use to encrypt the repo is the
same key used to sign it. If it's not, pulling from the repo won't work,
due to git-remote-gcrypt's "Only accepting signatories" check.

The user may have a global user.signingkey setting (I do), but be setting
up a different special-purpose key for encrypting their git repo. The
git-annex assistant cannot mess with the global value, so needs this to
override it.