replay: allow to specify a ref with option --ref

replay: allow to specify a ref with option --ref

When option '--onto' is passed to git-replay(1), the command will update
refs from the <revision-range> passed to the command. When using option
'--advance' or '--revert', the argument of that option is a ref that
will be updated.

To enable users to specify which ref to update, add option '--ref'. When
using option '--ref', the refs described above are left untouched and
instead the argument of this option is updated instead.

Because this introduces code paths in replay.c that jump to `out` before
init_basic_merge_options() is called on `merge_opt`, zero-initialize the
struct.

Signed-off-by: Toon Claes <toon@iotcl.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>