man: Use po4a “--previous” support when updating the man pages

Suggested-by: Christian Perrier <bubulle@debian.org>

debian/changelog

@@ -39,6 +39,8 @@ dpkg (1.15.4) UNRELEASED; urgency=low
     in dpkg man page.
   * Change po4a usage to not create unwated changes depending if doing out or
     in-tree builds.
+  * Use po4a “--previous” support when updating the man pages.
+    Suggested by Christian Perrier <bubulle@debian.org>.
 
   [ Raphael Hertzog ]
   * Replace install-info by a wrapper around GNU's install-info. The wrapper

man/Makefile.am

@@ -5,7 +5,7 @@ all-local: all-local-@USE_NLS@
 all-local-no:
 all-local-yes: man.stamp
 
-PO4A_COMMON_OPTS = --variable builddir=$(CURDIR)
+PO4A_COMMON_OPTS = --previous --variable builddir=$(CURDIR)
 PO4A_BUILD_OPTS = $(PO4A_COMMON_OPTS) --no-backups
 PO4A_CLEAN_OPTS = $(PO4A_COMMON_OPTS) --rm-backups --rm-translations