Show time since last change on index page

When creating the index page, an optional file can be scanned per repository
to obtain a timestamp for last modification within the repo. If such a file
cannot be found, st_mtime for repo.defbranch is used instead.

This information is then printed in a new column, "Idle", using the new
function cgit_print_age().

The new parameter "repo.agefile" can be used to specify (globally) a relative
path to scan (default value is "info/web/last-modified").

The content of the "last-modified" file can be generated by the post-receive
hook with a command like this:

	git-for-each-ref --format="%(committerdate)" --sort=-committerdate \
	--count=1 > $GIT_DIR/info/web/last-modified

Signed-off-by: Lars Hjemli <hjemli@gmail.com>