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()`

Fixed active_refspecs field not initialized on new git_remote objects

push: use the common refspec parser

Fixed a couple Clang warnings

When creating a new remote, contrary to loading one from disk,
active_refspecs was not populated. This means that if using the new
remote to push, git_push_update_tips() will be a no-op since it
checks the refspecs passed during the push against the base ones
i.e. active_refspecs. And therefore the local refs won't be created
or updated after the push operation.

There is one well-known and well-tested parser which we should use,
instead of implementing parsing a second time.

The common parser is also augmented to copy the LHS into the RHS if the
latter is empty.

The expressions test had to change a bit, as we now catch a bad RHS of a
refspec locally.