merge-ort: add pool_alloc, pool_calloc, and pool_strndup wrappers

Make the code more flexible so that it can handle both being run with or
without a memory pool by adding utility functions which will either call
    xmalloc, xcalloc, xstrndup
or
    mem_pool_alloc, mem_pool_calloc, mem_pool_strndup
depending on whether we have a non-NULL memory pool.  A subsequent
commit will make use of these.

(We will actually be dropping these functions soon and just assuming we
always have a memory pool, but the flexibility was very useful during
development of merge-ort so I want to be able to restore it if needed.)

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

merge-ort.c

@@ -664,6 +664,30 @@ static void path_msg(struct merge_options *opt,
 	strbuf_addch(sb, '\n');
 }
 
+MAYBE_UNUSED
+static void *pool_calloc(struct mem_pool *pool, size_t count, size_t size)
+{
+	if (!pool)
+		return xcalloc(count, size);
+	return mem_pool_calloc(pool, count, size);
+}
+
+MAYBE_UNUSED
+static void *pool_alloc(struct mem_pool *pool, size_t size)
+{
+	if (!pool)
+		return xmalloc(size);
+	return mem_pool_alloc(pool, size);
+}
+
+MAYBE_UNUSED
+static void *pool_strndup(struct mem_pool *pool, const char *str, size_t len)
+{
+	if (!pool)
+		return xstrndup(str, len);
+	return mem_pool_strndup(pool, str, len);
+}
+
 /* add a string to a strbuf, but converting "/" to "_" */
 static void add_flattened_path(struct strbuf *out, const char *s)
 {