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>

Author: derrickstolee

builtin/backfill.c

@@ -142,6 +142,9 @@ int cmd_backfill(int argc, const char **argv, const char *prefix, struct reposit
 	repo_init_revisions(repo, &ctx.revs, prefix);
 	argc = setup_revisions(argc, argv, &ctx.revs, NULL);
 
+	if (argc > 1)
+		die(_("unrecognized argument: %s"), argv[1]);
+
 	repo_config(repo, git_default_config, NULL);
 
 	if (ctx.sparse < 0)

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 --unexpected-arg --first-parent 2>err &&
+	test_grep "unrecognized 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 &&