dpkg-maintscript-helper: Support moving a conffile not being shipped

When the conffile stops being shipped, it might make sense to move it
somewhere else to preserve it. Do not fail if the new conffile does not
exist.

Closes: #767003
Signed-off-by: Guillem Jover <guillem@debian.org>

debian/changelog

@@ -94,6 +94,8 @@ dpkg (1.18.0) UNRELEASED; urgency=low
     - Improve available state sorting order strings in dselect panel.
   * Change dpkg-maintscript-helper to handle symlinks and pathnames ending in
     slash. For the former error out, for the latter strip it. Closes: #771752
+  * Support moving a conffile not being shipped anymore. Closes: #767003
+    Thanks to Mathias Behrle <mathiasb@m9s.biz>.
 
   [ Raphaël Hertzog ]
   * Drop myself from Uploaders.

scripts/dpkg-maintscript-helper.sh

@@ -210,7 +210,9 @@ finish_mv_conffile() {
 	ensure_package_owns_file "$PACKAGE" "$OLDCONFFILE" || return 0
 
 	echo "Preserving user changes to $NEWCONFFILE (renamed from $OLDCONFFILE)..."
-	mv -f "$NEWCONFFILE" "$NEWCONFFILE.dpkg-new"
+	if [ -e "$NEWCONFFILE" ]; then
+		mv -f "$NEWCONFFILE" "$NEWCONFFILE.dpkg-new"
+	fi
 	mv -f "$OLDCONFFILE" "$NEWCONFFILE"
 }