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 <b57189c5f2fd5b3daf3350a77fde90def1080fa5@gmail.com>
Reviewed-by: Ronnie Sahlberg <2cb6c6c4ccf58d41468189c2307f7a4ebf120324@google.com>
Signed-off-by: Junio C Hamano <a6723cc3f76163bf7adb636a73ac3b0ceb3e6b9b@pobox.com>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git