Noticed that when pairing, sometimes both sides start to push, and the other
side sends a PushRequest, and the two deadlock, neither doing anything.
(Timeout eventually breaks this.) So, let both run at the same time.
This should help prevent git-annex clients receiving messages that
were intended for normal clients they're sharing the account with.
Changed XMPP protocol use to always send chat messages directed at the
specific client, as the negative priority blocks less directed messages.
I decided to use the fallback push mode from the beginning for XMPP, since
while it uses some ugly branches, it avoids the possibility of a normal
push failing, and needing to pull and re-push. Due to the overhead of XMPP,
and the difficulty of building such a chain of actions due to the async
implementation, this seemed reasonable.
It seems to work great!
My reasoning is that StartingPush could be received after another push
starts being received, and it would be better to respond to it afterwards
than not.
XMPP has no defined message size limits, but some servers will have ad-hoc
limits. However, 4k seems safe, even after the additional bloat of base64.
That should not exceed 8k.
Inject the required git-remote-xmpp into PATH when running xmpp git push.
Rest of the time it will not be in PATH, and git won't be able to talk to
xmpp remotes.
It might even work, although nothing yet triggers XMPP pushes.
Also added a set of deferred push messages. Only one push can run at a
time, and unrelated push messages get deferred. The set will never grow
very large, because it only puts two types of messages in there, that
can only vary in the client doing the push.
