InitPluginStepsLoader: Ignore files prefixed .next_ and .last_

These files are ignored by PluginLoader when loading plugins in a
running site, but were not ignored when initializing a site.

As a result, if any of these files existing in /plugins had an init
step, the init step would be executed. This caused init to fail if
for example it was built against an incompatible Gerrit API version.

Change-Id: I03a2e0b9c730aaedc0b6899102b1f2f8ee17bbc4

gerrit-pgm/src/main/java/com/google/gerrit/pgm/init/InitPluginStepsLoader.java

@@ -122,7 +122,10 @@ public class InitPluginStepsLoader {
     DirectoryStream.Filter<Path> filter = new DirectoryStream.Filter<Path>() {
       @Override
       public boolean accept(Path entry) throws IOException {
-        return entry.getFileName().toString().endsWith(".jar")
+        String name = entry.getFileName().toString();
+        return name.endsWith(".jar")
+            && !name.startsWith(".last_")
+            && !name.startsWith(".next_")
             && Files.isRegularFile(entry);
       }
     };