dirstate: make restoring from backup avoid ambiguity of file stat
File .hg/dirstate is restored by renaming from backup in failure
inside scopes below. If renaming keeps ctime, mtime and size of a
file, restoring is overlooked, and old contents cached before
restoring isn't invalidated as expected.
- dirstateguard scope (from '.hg/dirstate.SUFFIX')
- transaction scope (from '.hg/journal.dirstate')
To avoid ambiguity of file stat at restoring, this patch invokes
vfs.rename() with checkambig=True.
This patch is a part of "Exact Cache Validation Plan":
https://www.mercurial-scm.org/wiki/ExactCacheValidationPlan