Rename ChangeField Directory

This name can be reserved by some configuration languages (e.g. SQL),
rename it to something, that is definately not reserverd.

Change-Id: Ia1b5700181e7acd97652c93ed484f2d355d9521e
Release-Notes: skip
diff --git a/java/com/google/gerrit/server/index/change/ChangeField.java b/java/com/google/gerrit/server/index/change/ChangeField.java
index bd93c6d..5b8d784 100644
--- a/java/com/google/gerrit/server/index/change/ChangeField.java
+++ b/java/com/google/gerrit/server/index/change/ChangeField.java
@@ -345,8 +345,7 @@
 
   /** Folders that are touched by the current patch set. */
   public static final IndexedField<ChangeData, Iterable<String>> DIRECTORY_FIELD =
-      IndexedField.<ChangeData>iterableStringBuilder("Directory")
-          .build(ChangeField::getDirectories);
+      IndexedField.<ChangeData>iterableStringBuilder("DirField").build(ChangeField::getDirectories);
 
   public static final IndexedField<ChangeData, Iterable<String>>.SearchSpec DIRECTORY_SPEC =
       DIRECTORY_FIELD.exact(ChangeQueryBuilder.FIELD_DIRECTORY);