install-webdoc: filter timestamp-only changes correctly

The timestamp that follows "Last updated " is formatted differently
depending on the version of AsciiDoc.  Looking at 4604fe56 on "html"
branch, you can see that AsciiDoc 7.0.2 used to give "02-Jul-2008 03:02:14
UTC" but AsciiDoc 8.2.5 gave "2008-09-19 06:33:25 UTC".  We haven't been
correctly filtering out phantom changes that result from only the build
date for some time now, it seems.

Just filter lines that begin with "Last updated ".

Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/install-webdoc.sh

@@ -12,7 +12,7 @@ do
 	then
 		: did not match
 	elif test -f "$T/$h" &&
-	   $DIFF -u -I'Last updated [0-9][0-9]-[A-Z][a-z][a-z]-' "$T/$h" "$h"
+		$DIFF -u -I'^Last updated ' "$T/$h" "$h"
 	then
 		:; # up to date
 	else