repository: Create ad-hoc remote with a random name

The FetchRemote RPC currently accepts either a remote name or a Remote
message as input: in the first case, it will try to fetch from a
preexisting remote, while in the latter case it will create an ad-hoc
remote and fetch from that one. Confusingly though, the user is supposed
to pass a remote name even for the ad-hoc repository: this is the name
used to create the remote as.

This remote name conveys the sense that it's used as a persistent
remote, but that's not the case: the remote is getting created and
deleted in the same call. Which effectively means that it doesn't
provide much of a benefit to the user in the first place, because why
would he care for the remote name if it doesn't persist anyway? The only
usecase one might think of is that it's done in order to set up the
default fetchspec of `refs/*:refs/remotes/$name/*`. But quite the
reverse: all refs which get fetched into the remote's namespace will get
deleted after the call. This is completely unexpected and quite harmful
because it may break mirror fetches it the remote happens to have refs
in there, too. Furthermore, if a persistent remote previously existed
from which we want to fetch, the caller will be sad because we simply
delete it.

Fix the issue by ignoring the user-provided name: instead, we now
generate a random remote name which is safe to get deleted afterwards.

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