如何读取git-ls-tree的输出模式字段
$ git ls-tree fb3a8bdd0ce 100644 blob 63c918c667fa005ff12ad89437f2fdc80926e21c .gitignore 100644 blob 5529b198e8d14decbe4ad99db3f7fb632de0439d .mailmap 100644 blob 6ff87c4664981e4397625791c8ea3bbb5f2279a3 COPYING 040000 tree 2fb783e477100ce076f6bf57e4a6f026013dc745 Documentation 100755 blob 3c0032cec592a765692234f1cba47dfdcc3a9200 GIT-VERSION-GEN 100644 blob 289b046a443c0647624607d471289b2c7dcd470b INSTALL 100644 blob 4eb463797adc693dc168b926b6932ff53f17d0b1 Makefile 100644 blob 548142c327a6790ff8821d67c2ee1eff7a656b52 README ... 我知道最后的3个数字是文件模式，但前3位数字是什么？ 我无法在git用户手册中find它。
6位数字使用传统的UNIX符号显示文件模式。 前两位数字显示文件types，第三位数字是关于set-uid / set-gid / sticky位，你知道最后三位。
下面是man 2 stat在我的GNU / Linux系统上的文档：
The following flags are defined for the st_mode field: S_IFMT 0170000 bit mask for the file type bit fields S_IFSOCK 0140000 socket S_IFLNK 0120000 symbolic link S_IFREG 0100000 regular file S_IFBLK 0060000 block device S_IFDIR 0040000 directory S_IFCHR 0020000 character device S_IFIFO 0010000 FIFO S_ISUID 0004000 set UID bit S_ISGID 0002000 set-group-ID bit (see below) S_ISVTX 0001000 sticky bit (see below) S_IRWXU 00700 mask for file owner permissions S_IRUSR 00400 owner has read permission S_IWUSR 00200 owner has write permission S_IXUSR 00100 owner has execute permission S_IRWXG 00070 mask for group permissions S_IRGRP 00040 group has read permission S_IWGRP 00020 group has write permission S_IXGRP 00010 group has execute permission S_IRWXO 00007 mask for permissions for others (not in group) S_IROTH 00004 others have read permission S_IWOTH 00002 others have write permission S_IXOTH 00001 others have execute permission
从Git的index-format.txt文件中，关于模式：
32-bit mode, split into (high to low bits) 4-bit object type valid values in binary are 1000 (regular file), 1010 (symbolic link) and 1110 (gitlink) 3-bit unused 9-bit unix permission. Only 0755 and 0644 are valid for regular files. Symbolic links and gitlinks have value 0 in this field.
另外，如fsck.c fsck_tree方法所示，允许目录对象types（二进制0100）和组可写（0664许可）常规文件。 常规的不可执行组可写文件是早期版本的Git支持的非标准模式。
这使得有效的模式（二进制和八进制）：
-
0100000000000000（040000）：目录 -
1000000110100100（100644）：普通的不可执行文件 -
1000000110110100（100664）：定期不可执行的可写组文件 -
1000000111101101（100755）：正100755执行文件 -
1010000000000000（120000）：符号链接 -
1110000000000000（160000）：Gitlink
