create_tmp_packfile(): a helper function

Factor out a small logic out of the private write_pack_file() function
in builtin/pack-objects.c

Signed-off-by: Junio C Hamano <gitster@pobox.com>

pack-write.c

@@ -328,3 +328,13 @@ int encode_in_pack_object_header(enum object_type type, uintmax_t size, unsigned
 	*hdr = c;
 	return n;
 }
+
+struct sha1file *create_tmp_packfile(char **pack_tmp_name)
+{
+	char tmpname[PATH_MAX];
+	int fd;
+
+	fd = odb_mkstemp(tmpname, sizeof(tmpname), "pack/tmp_pack_XXXXXX");
+	*pack_tmp_name = xstrdup(tmpname);
+	return sha1fd(fd, *pack_tmp_name);
+}