Avoid diff cost on "git log -z"

Johannes and Marco discovered that "git log -z" spent cycles in diff even
though there is no need to actually compute diffs.
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

revision.c

@@ -1171,7 +1171,8 @@ int setup_revisions(int argc, const char **argv, struct rev_info *revs, const ch
 
 			opts = diff_opt_parse(&revs->diffopt, argv+i, argc-i);
 			if (opts > 0) {
-				revs->diff = 1;
+				if (strcmp(argv[i], "-z"))
+					revs->diff = 1;
 				i += opts - 1;
 				continue;
 			}