title: git compiling: Documentation/git-add.xml does not validate
body: <p>When compiling git I have these errors:</p>
  
  <pre><code>make[2]: Leaving directory `/home/xxx/git-master'
      XMLTO git-add.1
  xmlto: /home/xxx/git-master/Documentation/git-add.xml does not validate (status 3)
  xmlto: Fix document syntax or use --skip-validation option
  I/O error : Attempt to load network entity http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd
  /home/xxx/git-master/Documentation/git-add.xml:2: warning: failed to load external entity "http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd"
  D DocBook XML V4.5//EN" "http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd"
  I/O error : Attempt to load network entity http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd
  warning: failed to load external entity "http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd"
  validity error : Could not load the external subset "http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd"
  Document /home/xxx/git-master/Documentation/git-add.xml does not validate
  make[1]: *** [git-add.1] Error 13
  make[1]: Leaving directory `/home/xxx/git-master/Documentation'
  make: *** [doc] Error 2
  </code></pre>
  
  <p>What is the main problem?</p>