changegroup: don't send empty subdirectory manifest groups When grafting/rebasing, it is common for multiple changesets to make the same change to a subdirectory. When writing the revlog for the directory, the revlog code already takes care of not writing the entry again. In 0c2a088ffcc5 (changegroup: prune subdirectory dirlogs too, 2016-02-12), I added the corresponding code in changegroup (not sending entries the client already has), but I forgot to avoid sending the entire changegroup if no nodes remained in the pruned set. Although that's harmless besides the wasted network traffic, the receiving side was checking for it (copied from the changegroup code for handling files). This resulted in the client crashing with: abort: received dir revlog group is empty Fix by simply not emitting a changegroup for the directory if there were no changes is it. This matches how files are handled.