.arcconfig

sshpeer: move URL validation out of sshpeer.__init__

We will soon have another SSH peer class to support the new version
of the SSH protocol. However, we won't know which peer class to
instantiate until we perform a handshake on an active connection.
This means that we need to move connection establishment and handshake
code out of sshpeer.__init__.

This commit starts the process of migrating peer creation code
out of sshpeer.__init__ into instance(), which is the API for
creating peers.

The moved code no longer calls _abort(). _abort() runs _cleanup() and
raises. _cleanup() only performs actions on self._pipe*. These objects
aren't instantiated until we actually connect to the peer. So _abort()
was not necessary in the old code.

To keep the API the same, __init__() now makes a redundant call to
util.url(). This will be fixed in subsequent commits.

Differential Revision: https://phab.mercurial-scm.org/D2027

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