install-webdoc: filter timestamp-only changes correctly

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>