Spelling fixes

peel: reject bad queries with EPEEL

There are some combination of objects and target types which we know
cannot be fulfilled. Return EINVALIDSPEC for those to signify that there
is a mismatch in the user-provided data and what the object model is
capable of satisfying.

If we start at a tag and in the course of peeling find out that we
cannot reach a particular type, we return EPEEL.

On disconnect we simply ask the transport to close the connection, we do
not free it.

Make it clear that this is not the ls-remote command but a way to access
the data we have and how long it's kept around.

Provide a convenience function `git_remote_push()`

If the user does not pass any refspecs to push, try to use those
configured via the configuration or via add_push().

This function, similar in style to git_remote_fetch(), performs all the
steps required for a push, with a similar interface.

The remote callbacks struct has learnt about the push callbacks, letting
us set the callbacks a single time instead of setting some in the remote
and some in the push operation.