format-patch: teach format.notes config option

format-patch: teach format.notes config option

In git-format-patch, notes can be appended with the `--notes` option.
However, this must be specified by the user on an
invocation-by-invocation basis. If a user is not careful, it's possible
that they may forget to include it and generate a patch series without
notes.

Teach git-format-patch the `format.notes` config option. Its value is a
notes ref that will be automatically appended. The special value of
"standard" can be used to specify the standard notes. This option is
overridable with the `--no-notes` option in case a user wishes not to
append notes.

Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>