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 to encode this behavior in test.

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

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>

Author: derrickstolee

t/t5620-backfill.sh

@@ -7,6 +7,14 @@ export GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME
 
 . ./test-lib.sh
 
+test_expect_success 'backfill rejects unexpected arguments' '
+	test_must_fail git backfill unexpected-arg 2>err &&
+	test_grep "ambiguous argument .*unexpected-arg" err &&
+
+	test_must_fail git backfill --all --firt-parent unexpected-arg 2>err &&
+	test_grep "ambiguous argument .*unexpected-arg" err
+'
+
 # We create objects in the 'src' repo.
 test_expect_success 'setup repo for object creation' '
 	echo "{print \$1}" >print_1.awk &&