t3404-rebase-interactive: test abbreviated commands

t3404-rebase-interactive: test abbreviated commands

Make sure that each short command is tested at least once. To
not exacerbate the runtime of the test script, do not add new
tests, but modify existing ones according to these criteria:

- The test does not have a prerequisite.
- The 'git rebase' command is not guarded by test_must_fail.

The pick commands are optional in the FAKE_LINES variable, but
when used, they do end up in the insn sheet. Test them, too.

Signed-off-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>