localrepo: make invalidate avoid invalidating store inside transaction (API) Before this patch, invalidate() discards in-memory fncache changes, even inside transaction scope. Such changes should be written out at closing transaction. Otherwise, fncache might overlook newly added files. A file overlooked by fncache isn't accessible via store vfs, even if it actually exists in store. On the other hand, a non-existing file in fncache is less harmful, because fncachestore always examines whether a file actually exists or not before access. Therefore, discarding in-memory changes can be safely omitted. It is typical case that repo.invalidate() in streamclone is executed inside nested transaction. This patch makes invalidate() avoid invalidating store inside transaction. This patch focuses on describing only how invalidate() changes own behavior according to activity of transaction. Describing other detail of invalidate() in docstr will be done in another series, which refactors invalidate*() functions.