po: Fix copy of po file from underlay when editing

When first editing a page that was in the underlay, avoid losing
the translation by copying the po file over from the underlay.

IkiWiki/Plugin/po.pm

@@ -827,6 +827,21 @@ sub refreshpofiles ($@) {
 
 	foreach my $pofile (@pofiles) {
 		IkiWiki::prep_writefile(basename($pofile),dirname($pofile));
+
+		if (! -e $pofile) {
+			# If the po file exists in an underlay, copy it
+			# from there.
+			my ($pobase)=$pofile=~/^\Q$config{srcdir}\E\/?(.*)$/;
+			foreach my $dir (@{$config{underlaydirs}}) {
+				if (-e "$dir/$pobase") {
+					File::Copy::syscopy("$dir/$pobase",$pofile)
+						or error("po(refreshpofiles) ".
+							 sprintf(gettext("failed to copy underlay PO file to %s"),
+								 $pofile));
+				}
+			}
+		}
+
 		if (-e $pofile) {
 			system("msgmerge", "--previous", "-q", "-U", "--backup=none", $pofile, $potfile) == 0
 				or error("po(refreshpofiles) ".

doc/plugins/po.mdwn

@@ -277,17 +277,6 @@ That used to be so, but the bug was fixed. Does this mean
 that po might be replacing the only link on a page, in error? 
 --[[Joey]] 
 
-Bug when editing underlay file
-------------------------------
-
-While I've gotten translated underlays working, there is a bug
-if the wiki is currently using a page from the underlay, and the master
-language version is edited. This causes the edited page to be saved
-to srcdir.. and all the translations get set to 0% and their
-po files have the translated strings "emptied". What's really going on
-is that these are entirely new po files not based on the old ones
-in the basewiki, and thus lacking translations. --[[Joey]] 
-
 Documentation
 -------------