MyFirstContribution: teach to use "format-patch --base=auto"

Let's encourage first-time contributors to tell us what commit they
based their work on with the format-patch invocation.  As the
example already forks from origin/master and branch.autosetupmerge
by default records the upstream when the psuh branch was created, we
can use --base=auto for this.  Also, mention that the range of
commits can simply be given with `@{u}` if they are on the `psuh`
branch already.

As we are getting one more option on the command line, and spending
one paragraph each to explain them, let's reformat that part of the
description as a bulleted list.

Helped-by: Bagas Sanjaya <bagasdotme@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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