Fix mergeJar prefix sometimes adding an extra / to file names when reading from directories.

Author: LexManos

file-utils/src/main/java/net/minecraftforge/util/file/FileUtils.java

@@ -162,6 +162,8 @@ public static void mergeJars(File output, boolean stripSignatures, @Nullable BiP
 
                 if (input.isDirectory()) {
                     var prefix = input.getAbsolutePath();
+                    if (!prefix.endsWith(File.separator))
+                        prefix += File.separatorChar;
                     for (var file : FileUtils.listFiles(input)) {
                         var name = file.getAbsolutePath().substring(prefix.length()).replace('\\', '/');