rev-list --header: output format fix

Initial fix prepared by Johannes, but I did it slightly differently.
Signed-off-by: NJunio C Hamano <junkio@cox.net>

rev-list.c

@@ -326,6 +326,9 @@ int main(int argc, const char **argv)
 		else
 			revs.header_prefix = "commit ";
 	}
+	else if (revs.verbose_header)
+		/* Only --header was specified */
+		revs.commit_format = CMIT_FMT_RAW;
 
 	list = revs.commits;