Add new persistent cache "diff_file_path_list" to cache file paths only

The new cache caches list of paths of changed files. Main intention of
the new cache is to speed up change reindexing. By caching list of
changed files we avoid computing diff for that purpose.

NOTE: The existing "diff" cache implicitly caches list of changed files
but its entries are quite large to keep it populated for all changes.

Change-Id: I45f90deeba9fe70f32e4b0a5657bfb211ecc2eeb

gerrit-server/src/main/java/com/google/gerrit/server/patch/PatchListCache.java

@@ -33,4 +33,7 @@ public interface PatchListCache {
 
   IntraLineDiff getIntraLineDiff(IntraLineDiffKey key,
       IntraLineDiffArgs args);
+
+  FileList getFileList(Change change, PatchSet patchSet)
+      throws PatchListNotAvailableException;
 }