Sometimes a name has been used for a special remote, and you want to change
the name. A common reason is that the special remote has become dead, and
you want to reuse the name for a new special remote. 

Initremote prevents reusing a name when the old one exists, even if the old
one is dead. And that makes sense in general, because a dead remote can
come back sometimes, and that would leave the repo with two special remotes
with the same name, and so enableremote would need to be run with a uuid
instead of a name to specify which one to enable, which is not a desirable
state of affairs.

So, add `git annex renameremote oldname newname`. This could also do a `git
remote rename`, or equivilant. (`git remote rename` gets confused by special
remotes not having a fetch url and fails; this can be worked around by
manually renaming the stanza in git config.)

Implementing that would need a way to remove the old name from remote.log.
We can't remove lines from union merged files, but what we could do is
add a new line like:

	- name=oldname timestamp=<latest>

And in parsing remote.log, if the UUID is "-", don't include the
remote with that name in the the resulting map.