title: How to read the mode field of git-ls-tree's output
body: <pre><code>$ 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
  ...
  </code></pre>
  
  <p>I know the last 3 oct digits are file mode, but what are the first 3 digits for?
  I can't find it out in git user's manual.</p>