doc: replace git config --list/-l with `list`

doc: replace git config --list/-l with `list`

Replace uses of `git config --list` (short or long) with the subcommand
`list` since `--list` is deprecated.

We will change the “man page” phrasing in gitcvs-migration(7) in the
next commit, since we are already visiting that sentence. But note
that we leave the “man page” phrasing in the sentence that we touch in
gittutorial(7) since it’s a tutorial and not a manual page. We can be
more wordy in a tutorial context.

Signed-off-by: Kristoffer Haugsbakk <code@khaugsbakk.name>
Signed-off-by: Junio C Hamano <gitster@pobox.com>