push: refactor refspec_append_mapped() for subsequent leak-fix

push: refactor refspec_append_mapped() for subsequent leak-fix

The set_refspecs() caller of refspec_append_mapped() (added in [1])
left open the question[2] of whether the "remote" we lazily fetch
might be NULL in the "[...]uniquely name our ref?" case, as
remote_get() can return NULL.

If we got past the "[...]uniquely name our ref?" case we'd have
already segfaulted if we tried to dereference it as
"remote->push.nr". In these cases the config mechanism & previous
remote validation will have bailed out earlier.

Let's refactor this code to clarify that, we'll now BUG() out if we
can't get a "remote", and will no longer retrieve it for these common
cases where we don't need it.

1. ca02465b413 (push: use remote.$name.push as a refmap, 2013-12-03)
2. https://lore.kernel.org/git/c0c07b89-7eaf-21cd-748e-e14ea57f09fd@web.de/

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>