dirstate: add identity information to detect simultaneous changing in storage This identity is used to examine whether dirstate is simultaneously changed in storage after previous caching (see issue5584 for detail). util.cachestat can't be used for this purpose, because it has no valuable information on Windows. On the other hand, util.filestat can detect changing dirstate in storage certainly, regardless of platforms. https://www.mercurial-scm.org/wiki/ExactCacheValidationPlan Strictly speaking, if underlying filesystem doesn't support ctime/mtime, util.filestat can't detect simultaneous changing in storage as expected. But simultaneous changing on such (very rare) platform can't be detected regardless of this patch series. Therefore, util.filestat should be reasonable identity for almost all usecases.