refs.c: do not permit err == NULL

refs.c: do not permit err == NULL

Some functions that take a strbuf argument to append an error treat
!err as an indication that the message should be suppressed (e.g.,
ref_update_reject_duplicates).  Others write the message to stderr on
!err (e.g., repack_without_refs).  Others crash (e.g.,
ref_transaction_update).

Some of these behaviors are for historical reasons and others were
accidents.  Luckily no callers pass err == NULL any more.  Simplify
by consistently requiring the strbuf argument.

Signed-off-by: Jonathan Nieder <redacted>
Reviewed-by: Ronnie Sahlberg <redacted>
Signed-off-by: Junio C Hamano <redacted>