sync: Fix parsing of gcrypt::rsync:// urls that use a relative path

sync: Fix parsing of gcrypt:: urls that use a relative path

Such an url is not valid; parseURI will fail on it. But git-annex doesn't
actually need to parse the url, because all it needs to do to support
syncing with it is know that it's not a local path, and use git pull and
push.

(Note that there is no good reason for the user to use such an url. An
absolute url is valid and I patched git-remote-gcrypt to support them
years ago. Still, users gonna do anything that tools allow, and
git-remote-gcrypt still supports them.)

Sponsored-by: Jack Hill on Patreon