porters-handbook: Fix the Git process in Chapter 11.1

In Chapter 11.1 Using Git to Make Patches, there are multiple issues
with the process:

• git pull --rebase will refuse to work if there are any staged or unstaged changes, so the users are unable to update the repo to the latest.
• git diff does not work for staged changes, need to add --staged.

This commit attempts to address the above.

PR: 256122
Reported by: Neal Nelson <ports@nicandneal.net>
Reviewed by: bcr, PauAmma <pauamma@gundo.com>
Differential Revision: https://reviews.freebsd.org/D30422