SubmittingPatches: spell out "replace fully to pretend to be perfect"

SubmittingPatches: spell out "replace fully to pretend to be perfect"

It unfortunately is a recurring theme that new developers tend to
pile more "fixup" patches on top of the already reviewed patches,
making the topic longer and keeping the history of all wrong turns,
which interests nobody in the larger picture.  Even picking a narrow
search in the list archive for "pretend to be a perfect " substring,
we find these:

    https://lore.kernel.org/git/xmqqk29bsz2o.fsf@gitster.mtv.corp.google.com/
    https://lore.kernel.org/git/xmqqd0ds5ysq.fsf@gitster-ct.c.googlers.com/
    https://lore.kernel.org/git/xmqqr173faez.fsf@gitster.g/

The SubmittingPatches guide does talk about going incremental once a
topic hits the 'next' branch, but it does not say much about how a
new iteration of the topic should be prepared before that happens,
and it does not mention that the developers are encouraged to seize
the opportunity to pretend to be perfect with a full replacement set
of patches.

Add a new paragraph to stress this point in the section that
describes the life-cycle of a patch series.

Signed-off-by: Junio C Hamano <gitster@pobox.com>