MyFirstContribution: refrain from self-iterating too much

MyFirstContribution: refrain from self-iterating too much

Finding mistakes in and improving your own patches is a good idea,
but doing so too quickly is being inconsiderate to reviewers who
have just seen the initial iteration and taking their time to review
it.  Encourage new developers to perform such a self review before
they send out their patches, not after.  After sending a patch that
they immediately found mistakes in, they are welcome to comment on
them, mentioning what and how they plan to improve them in an
updated version, before sending out their updates.

Helped-by: Torsten Bögershausen <tboegi@web.de>
Helped-by: Linus Arver <linusa@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>