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.

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.

This describes their purpose better, as we now initialize ssl and some
other global stuff in there. Calling the init function is not something
which has been optional for a while now.

Refactor fetchhead

remote: rename _load() to _lookup()

This gets the value from branch.<foo>.remote.

This brings it in line with the rest of the lookup functions.