subrepo: implement "storeclean" method for mercurial subrepos The mercurial subrepo "storeclean" method works by calculating a "store hash" of the repository state and comparing it to a cached store hash. The store hash is always cached when the repository is cloned from or pushed to a remote repository, but also on pull as long as the repository already had a clean store. If the hashes match the store is "clean" versus the selected repository. Note that this method is currenty unused, but it will be used by a later patch. The store hash is calculated by hashing several key repository files, such as the bookmarks file the phaseroots file and the changelog. Note that the hash comparison is done file by file so that we can exit early if a pair of hashes do not match. Also the hashes are calculated starting with the file that is most likely to be smaller upto the file that is more likely to be larger.