revision.c: stricter parsing of '--early-output'

revision.c: stricter parsing of '--early-output'

The parsing of '--early-output' with or without its optional integer
argument allowed bogus options like '--early-output-foobarbaz' to slip
through and be ignored.

Fix it by parsing '--early-output' in the same way as other options
with an optional argument are parsed.  Furthermore, use strtoul_ui()
to parse the optional integer argument and to refuse negative numbers.

While at it, use skip_prefix() instead of starts_with() and magic
numbers.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Reviewed-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>