doc: set actual revdate for manpages

doc: set actual revdate for manpages

manpages expect the date of the last revision, if that is not found
DocBook Stylesheets go through a series of hacks to generate one with
the format `%d/%d/%Y` which is not ideal.

In addition to this format not being standard, different tools generate
dates with different formats.

There's no need for any confusion if we specify the revision date, so
let's do so.

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>