Show time since last change on index page

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>