po4a --previous option in release-notes

[Simon Paillard <simon.paillard@resel.enst-bretagne.fr>]

Hi Martin,

By running updatepo this evening, I noticed we lost the opportunity to
show the previous version of the string to translate.

It seems you reverted the change I made in the makefile :
#UPDATEPO=PERLLIB=lib/ po4a-updatepo --previous --format docbook
UPDATEPO=PERLLIB=lib/ po4a-updatepo --format docbook 

I don't see any good reason to miss the --previous feature, which is
*much* useful for translators.

Can we enable it back ?

-- 
Simon Paillard