transaction: open a file with checkambig=True to avoid file stat ambiguity Before this patch, if steps below occurs at "the same time in sec", all of mtime, ctime and size are same between (1) and (3). 1. append data to revlog-style file (and close transaction) 2. discard appended data by truncation of rollback 3. append same size but different data to revlog-style file again Therefore, cache validation doesn't work after (3) as expected. To avoid file stat ambiguity around truncation, this patch opens a file with checkambig=True. This is a part of ExactCacheValidationPlan. https://www.mercurial-scm.org/wiki/ExactCacheValidationPlan