gitweb.js: Harden setting blamed commit info in incremental blame

gitweb.js: Harden setting blamed commit info in incremental blame

Internet Explorer 8 stops at beginning of blame filling with the
following bug:

  "firstChild is null or not an object"

at this line:

  a_sha1.firstChild.data = commit.sha1.substr(0, 8);

It is (probably) caused by the fact that while a_sha1 element, which
looks like this:

  <a href=""> </a>

It has a firstChild which is a text node containing only whitespace
(single space character) in other web browsers (Firefox 3.5, Opera 10,
Google Chrome 3.0), IE8 clobbers DOM, removing trailing/leading
whitespace.

Protect against this bug by creating text element if it does not
exist.

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