util: add filestat class to detect ambiguity of file stat Current posix.cachestat implementation might overlook change of a file, if changing keeps ctime, mtime and size of file. Comparison of inode number also overlooks changing in such situation, because inode number is rapidly reused. Contents of a file cached before changing isn't invalidated as expected, if change of a file is overlooked for this "ambiguity" of file stat. This patch adds filestat class to detect ambiguity of file stat. This patch is a part of preparation for "Exact Cache Validation Plan": https://www.mercurial-scm.org/wiki/ExactCacheValidationPlan