[file-history, hg] return repository current revision for directories

Asking for a directory specifically returns null, which breaks actions such as "Compare with Local".

Change was made in a37c643781 in order to reuse calcCurrentRevisionNumber in VcsCachingHistory instead of calling a diff provider. But that was not actually necessary, since mercurial file history does not use cache at all.

IDEA-196914

plugins/hg4idea/src/org/zmlx/hg4idea/provider/HgHistoryProvider.java

@@ -111,6 +111,7 @@ public class HgHistoryProvider implements VcsHistoryProvider {
       @Override
       @Nullable
       protected VcsRevisionNumber calcCurrentRevisionNumber() {
+        if (filePath.isDirectory()) return new HgWorkingCopyRevisionsCommand(myProject).firstParent(vcsRoot);
         return new HgWorkingCopyRevisionsCommand(myProject).parents(vcsRoot, filePath).first;
       }