remote set-head: refactor for readability

remote set-head: refactor for readability

Make two different readability refactors:

Rename strbufs "buf" and "buf2" to something more explanatory.

Instead of calling get_main_ref_store(the_repository) multiple times,
call it once and store the result in a new refs variable. Although this
change probably offers some performance benefits, the main purpose is to
shorten the line lengths of function calls using this variable.

Signed-off-by: Bence Ferdinandy <bence@ferdinandy.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>