shortlog: support outputting to streams other than stdout

This will be needed to avoid freopen() in `git format-patch`.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/shortlog.c

@@ -229,6 +229,7 @@ void shortlog_init(struct shortlog *log)
 	log->wrap = DEFAULT_WRAPLEN;
 	log->in1 = DEFAULT_INDENT1;
 	log->in2 = DEFAULT_INDENT2;
+	log->file = stdout;
 }
 
 int cmd_shortlog(int argc, const char **argv, const char *prefix)
@@ -310,22 +311,24 @@ void shortlog_output(struct shortlog *log)
 	for (i = 0; i < log->list.nr; i++) {
 		const struct string_list_item *item = &log->list.items[i];
 		if (log->summary) {
-			printf("%6d\t%s\n", (int)UTIL_TO_INT(item), item->string);
+			fprintf(log->file, "%6d\t%s\n",
+				(int)UTIL_TO_INT(item), item->string);
 		} else {
 			struct string_list *onelines = item->util;
-			printf("%s (%d):\n", item->string, onelines->nr);
+			fprintf(log->file, "%s (%d):\n",
+				item->string, onelines->nr);
 			for (j = onelines->nr - 1; j >= 0; j--) {
 				const char *msg = onelines->items[j].string;
 
 				if (log->wrap_lines) {
 					strbuf_reset(&sb);
 					add_wrapped_shortlog_msg(&sb, msg, log);
-					fwrite(sb.buf, sb.len, 1, stdout);
+					fwrite(sb.buf, sb.len, 1, log->file);
 				}
 				else
-					printf("      %s\n", msg);
+					fprintf(log->file, "      %s\n", msg);
 			}
-			putchar('\n');
+			putc('\n', log->file);
 			onelines->strdup_strings = 1;
 			string_list_clear(onelines, 0);
 			free(onelines);

shortlog.h

@@ -17,6 +17,7 @@ struct shortlog {
 	char *common_repo_prefix;
 	int email;
 	struct string_list mailmap;
+	FILE *file;
 };
 
 void shortlog_init(struct shortlog *log);