reflog: convert to parse_options() API

Continue the work started in 33d7bdd645 (builtin/reflog.c: use
parse-options api for expire, delete subcommands, 2022-01-06) and
convert the cmd_reflog() function itself to use the parse_options()
API.

Let's also add a test which would fail if we forgot
PARSE_OPT_NO_INTERNAL_HELP here, as well as making sure that we'll
still pass through "--" by supplying PARSE_OPT_KEEP_DASHDASH. For that
test we need to change "test_commit()" to accept files starting with
"--".

The "git reflog -h" usage will now show the usage for all of the
sub-commands, rather than a terse summary which wasn't
correct (e.g. "git reflog exists" is not a valid command). See my
8757b35d44 (commit-graph: define common usage with a macro,
2021-08-23) for prior art.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/test-lib-functions.sh

@@ -329,7 +329,7 @@ test_commit () {
 	else
 		$echo "${3-$1}" >"$indir$file"
 	fi &&
-	git ${indir:+ -C "$indir"} add "$file" &&
+	git ${indir:+ -C "$indir"} add -- "$file" &&
 	if test -z "$notick"
 	then
 		test_tick