revision: add rdiff_log_arg to rev_info

revision: add rdiff_log_arg to rev_info

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`. This is handled in
`builtin/log.c` by the local variable `log_arg` in the case of mul-
tiple commits, but not in the single commit case where there is no
cover letter and the range diff is embedded in the patch output; the
range diff is then made in `log-tree.c`, whither `log_arg` has not
been propagated. This means that the range-diff subprocess reverts
to its default behavior, which is to act like git-log(1) w.r.t. notes.

We need to fix this. But first lay the groundwork by converting
`log_arg` to a struct member; next we can simply use that member
in `log-tree.c` without having to thread it from `builtin/log.c`.

No functional changes.

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