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: NStephen Boyd <bebarino@gmail.com>
Signed-off-by: NJakub Narebski <jnareb@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

gitweb/gitweb.js

@@ -562,7 +562,12 @@ function handleLine(commit, group) {
 			td_sha1.rowSpan = group.numlines;
 
 			a_sha1.href = projectUrl + 'a=commit;h=' + commit.sha1;
-			a_sha1.firstChild.data = commit.sha1.substr(0, 8);
+			if (a_sha1.firstChild) {
+				a_sha1.firstChild.data = commit.sha1.substr(0, 8);
+			} else {
+				a_sha1.appendChild(
+					document.createTextNode(commit.sha1.substr(0, 8)));
+			}
 			if (group.numlines >= 2) {
 				var fragment = document.createDocumentFragment();
 				var br   = document.createElement("br");