Add comment to explain semantics of file content validation

Change-Id: Ib24d74ac949d456d5bde86aad2ca76d635ed2e77

src/main/java/com/googlesource/gerrit/plugins/uploadvalidator/CommitUtils.java

diff --git a/src/main/java/com/googlesource/gerrit/plugins/uploadvalidator/CommitUtils.java b/src/main/java/com/googlesource/gerrit/plugins/uploadvalidator/CommitUtils.java
index 5505343..c589586 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/uploadvalidator/CommitUtils.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/uploadvalidator/CommitUtils.java
@@ -90,6 +90,10 @@
    * If a TreeWalk entry is found this method calls the onVisit() method of the class
    * TreeWalkVisitor.
    *
+   * <p>We assume that all parents have been validated successfully. If the commit is identical to
+   * one of them, we consider it valid, too, even if e.g. validation rules have since changed or
+   * differ from the ones applicable to the target branch.
+   *
    * @param repo The repository
    * @param c The commit
    * @param visitor A TreeWalkVisitor with the desired action