diff-lib: tighten show_interdiff()'s interface

To compute and show an interdiff, show_interdiff() needs only the two
OID's to compare and a diffopts, yet it expects callers to supply an
entire rev_info. The demand for rev_info is not only overkill, but also
places unnecessary burden on potential future callers which might not
otherwise have a rev_info at hand. Address this by tightening its
signature to require only the items it needs instead of a full rev_info.

Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/log.c

@@ -1206,7 +1206,8 @@ static void make_cover_letter(struct rev_info *rev, int use_stdout,
 
 	if (rev->idiff_oid1) {
 		fprintf_ln(rev->diffopt.file, "%s", rev->idiff_title);
-		show_interdiff(rev, 0);
+		show_interdiff(rev->idiff_oid1, rev->idiff_oid2, 0,
+			       &rev->diffopt);
 	}
 
 	if (rev->rdiff1) {