build/i18n-merge-master: ignore apps deleted in master

Refine the code to ignore apps that have been deleted from master
but still exist in release branches. E.g. luci-app-samba

Previously the unhandled git error from non-existing master
mangled the .po files in the release branch: the 18n header was
removed and all non-ASCII chars were deleted from translation.

Fix this by processing only those files where 'git show' succeeds.

Signed-off-by: Hannu Nyman <hannu.nyman@iki.fi>

(cherry picked from commit e4baee8d25 in openwrt-19.07)

build/i18n-merge-master.pl

@@ -7,9 +7,14 @@ if (open F, '-|', 'find', $ARGV[0] || '.', '-type', 'f', '-name', '*.po') {
 		(my $ref = $path) =~ s/\.po$/\.master.po/;
 
 		printf 'Updating %s ', $path;
-		system("git show --format=\%B 'master:$path' > '$ref'");
-		system('msgmerge', '-N', '-o', $path, $ref, $path);
-		system('msgattrib', '--no-obsolete', '-o', $path, $path);
+		my $returnCode = system("git show --format=\%B 'master:$path' > '$ref'");
+		if ( $returnCode == 0 )
+		{
+			system('msgmerge', '-N', '-o', $path, $ref, $path);
+			system('msgattrib', '--no-obsolete', '-o', $path, $path);
+		} else {
+			print "...failed due to git error.\n";
+		}
 		unlink($ref);
 	}