add comments

Author: Vladimir Kotal

opengrok-indexer/src/main/java/org/opengrok/indexer/history/MercurialHistoryParser.java

@@ -99,7 +99,7 @@ History parse(File file, String sinceRevision, String tillRevision) throws Histo
             repository.removeAndVerifyOldestChangeset(entries, sinceRevision);
         }
 
-        // TODO: add comment
+        // See getHistoryLogExecutor() for explanation.
         if (repository.isHandleRenamedFiles() && file.isFile() && tillRevision != null) {
             removeChangesets(entries, tillRevision);
         }

opengrok-indexer/src/main/java/org/opengrok/indexer/history/MercurialRepository.java

@@ -224,7 +224,8 @@ Executor getHistoryLogExecutor(File file, String sinceRevision, String tillRevis
             // Getting history for individual files should only be done when generating history for renamed files
             // so the fact that filtering on sinceRevision does not work does not matter there as history
             // from the initial changeset is needed. The tillRevision filtering works however not
-            // in combination with --follow.
+            // in combination with --follow so the filtering is done in MercurialHistoryParser.parse().
+            // Even if the revision filtering worked, this approach would be probably faster and consumed less memory.
             if (this.isHandleRenamedFiles()) {
                 // When using --follow, the returned revisions are from newest to oldest, hence no reverse() is needed.
                 cmd.add("--follow");