t5620: test backfill's unknown argument handling

t5620: test backfill's unknown argument handling

Before the recent changes to parse rev-list arguments inside of 'git
backfill', the builtin would take arbitrary arguments without complaint (and
ignore them). This was noticed and a patch was sent [1] which motivates
this change.

[1] https://lore.kernel.org/git/20260321031643.5185-1-r.siddharth.shrimali@gmail.com/

Note that the revision machinery can output an "ambiguous argument"
warning if a value not starting with '--' is found and doesn't make
sense as a reference or a pathspec. For unrecognized arguments starting
with '--' we need to add logic into builtin/backfill.c to catch leftover
arguments.

Reported-by: Siddharth Shrimali <r.siddharth.shrimali@gmail.com>
Signed-off-by: Derrick Stolee <stolee@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>