fast-import: use mem_pool_calloc()

Use mem_pool_calloc() to get a zeroed buffer instead of zeroing it
ourselves.  This makes the code clearer and less repetitive.

Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: rscharfe

builtin/fast-import.c

@@ -2809,8 +2809,7 @@ static void parse_new_tag(const char *arg)
 	enum object_type type;
 	const char *v;
 
-	t = mem_pool_alloc(&fi_mem_pool, sizeof(struct tag));
-	memset(t, 0, sizeof(struct tag));
+	t = mem_pool_calloc(&fi_mem_pool, 1, sizeof(struct tag));
 	t->name = mem_pool_strdup(&fi_mem_pool, arg);
 	if (last_tag)
 		last_tag->next_tag = t;