gitweb.js: Workaround for IE8 bug

gitweb.js: Workaround for IE8 bug

In Internet Explorer 8 (IE8) the 'blame_incremental' view, which uses
JavaScript to generate blame info using AJAX, sometimes hang at the
beginning (at 0%) of blaming, e.g. for larger files with long history
like git's own gitweb/gitweb.perl.

The error shown by JavaScript console is "Unspecified error" at char:2
of the following line in gitweb/gitweb.js:

  if (xhr.readyState === 3 && xhr.status !== 200) {

Debugging it using IE8 JScript debuger shown that the error occurs
when trying to access xhr.status (xhr is XMLHttpRequest object).
Watch for xhr object shows 'Unspecified error.' as "value" of
xhr.status, and trying to access xhr.status from console throws error.

This bug is some intermittent bug, depending on XMLHttpRequest timing,
as it doesn't occur in all cases.  It is probably caused by the fact
that handleResponse is called from timer (pollTimer), to work around
the fact that some browsers call onreadystatechange handler only once
for each state change, and not like required for 'blame_incremental'
as soon as new data is available from server.  It looks like xhr
object is not properly initialized; still it is a bug to throw an
error when accessing xhr.status (and not use 'null' or 'undefined' as
value).

Work around this bug in IE8 by using try-catch block when accessing
xhr.status.

Signed-off-by: Jakub Narebski <jnareb@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>