format-patch: handle range-diff on notes correctly for single patches

(The two next paragraphs are taken from the previous commit.)

git-format-patch(1) supports Git notes by showing them beneath the
patch/commit message, similar to git-log(1). The command also supports
showing those same notes ref names in the range diff output.

Note *the same* ref names; any Git notes options or configuration
variables need to be handed off to the range-diff machinery. This works
correctly in the case when the range diff is on the cover letter. But it
does not work correctly when the output is a single patch with an
embedded range diff.

Concretely, git-format-patch(1) needs to pass `--[no-]notes` options on
to the range-diff subprocess in `range-diff.c`. Range diffs for single-
commit series are handled in `log-tree.c`. But `log-tree.c` had no
access to any `log_arg` variable before we added it to `rev_info` in the
previous commit.

Use that new struct member to fix this inconsistency.

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