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>

log-tree.c

@@ -718,7 +718,8 @@ static void show_diff_of_diff(struct rev_info *opt)
 			.creation_factor = opt->creation_factor,
 			.dual_color = 1,
 			.max_memory = RANGE_DIFF_MAX_MEMORY_DEFAULT,
-			.diffopt = &opts
+			.diffopt = &opts,
+			.log_arg = &opt->rdiff_log_arg
 		};
 
 		memcpy(&dq, &diff_queued_diff, sizeof(diff_queued_diff));

t/t3206-range-diff.sh

@@ -707,7 +707,7 @@ test_expect_success 'format-patch --range-diff does not compare notes by default
 	! grep "note" 0000-*
 '
 
-test_expect_success 'format-patch --notes=custom --range-diff only compares custom notes' '
+test_expect_success 'format-patch --notes=custom --range-diff --cover-letter only compares custom notes' '
 	test_when_finished "git notes remove topic unmodified || :" &&
 	git notes add -m "topic note" topic &&
 	git notes add -m "unmodified note" unmodified &&
@@ -721,6 +721,20 @@ test_expect_success 'format-patch --notes=custom --range-diff only compares cust
 	! grep "## Notes ##" 0000-*
 '
 
+# --range-diff on a single commit requires --no-cover-letter
+test_expect_success 'format-patch --notes=custom --range-diff on single commit only compares custom notes' '
+	test_when_finished "git notes remove HEAD unmodified || :" &&
+	git notes add -m "topic note" HEAD &&
+	test_when_finished "git notes --ref=custom remove HEAD unmodified || :" &&
+	git notes add -m "unmodified note" unmodified &&
+	git notes --ref=custom add -m "topic note (custom)" HEAD &&
+	git notes --ref=custom add -m "unmodified note (custom)" unmodified &&
+	git format-patch --notes=custom --range-diff=$prev \
+		-1 --stdout >actual &&
+	test_grep "## Notes (custom) ##" actual &&
+	test_grep ! "## Notes ##" actual
+'
+
 test_expect_success 'format-patch --range-diff with --no-notes' '
 	test_when_finished "git notes remove topic unmodified || :" &&
 	git notes add -m "topic note" topic &&