refs: factor out set_read_ref_cutoffs()

refs: factor out set_read_ref_cutoffs()

This block of code is duplicated twice. In a future commit, it will be
duplicated for a third time. Factor out the common functionality into
set_read_ref_cutoffs().

In the case of read_ref_at_ent(), we are incrementing `cb->reccnt` at the
beginning of the function. Move these to right before the return so that
the `cb->reccnt - 1` is changed to `cb->reccnt` and it can be cleanly
factored out into set_read_ref_cutoffs(). The duplication of the
increment statements will be removed in a future patch.

Signed-off-by: Denton Liu <redacted>
Signed-off-by: Junio C Hamano <redacted>