update-ref: pass end pointer instead of strbuf

update-ref: pass end pointer instead of strbuf

We currently pass both an `strbuf` containing the current command line
as well as the `next` pointer pointing to the first argument to
commands. This is both confusing and makes code more intertwined.
Convert this to use a simple pointer as well as a pointer pointing to
the end of the input as a preparatory step to line-wise reading of
stdin.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>