doc/man: use specific revision date for each manual page

This patch makes each manual page indicate its own revision date with
the `revdate` AsciiDoc attribute.

In `asciidoc.conf`, we use this attribute to specify the DocBook
reference page date (see
<https://tdg.docbook.org/tdg/4.5/refentryinfo.html> and
<https://tdg.docbook.org/tdg/4.5/date.html>).

Without the DocBook date tag, `xmlto` uses the current date. You can
see this date at the bottom of the rendered manual page:

    ...

    SEE ALSO
           lttng-enable-rotation(1), lttng-disable-rotation(1), lttng(1)

    LTTng 2.12.0-pre             10/18/2019              LTTNG-ROTATE(1)

Using the manual page generation date seems unexpected for the reader
here.

For this initial change, I used the last commit date for each source
file.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>