shelve: keep old backups if timestamp can't decide exact order of them Before this patch, backups to be discarded are decided by steps below at 'hg unshelve' or so: 1. list '(st_mtime, filename)' tuples of each backups up 2. sort list of these tuples, and 3. discard backups other than 'maxbackups' ones at the end of list This doesn't work well in the case below: - "sort by name" order differs from actual backup-ing order, and - some of backups have same timestamp For example, 'test-shelve.t' satisfies the former condition: - 'default-01' < 'default-1' in "sort by name" order - 'default-1' < 'default-01' in actual backup-ing order Then, 'default-01' is discarded instead of 'default-1' unexpectedly, if they have same timestamp. This failure appears occasionally, because the most important condition "same timestamp" is timing critical. To avoid such unexpected discarding, this patch keeps old backups if timestamp can't decide exact order of them. Timestamp of the border backup (= the oldest one of recent 'maxbackups' ones) as 'bordermtime' is used to examine whether timestamp can decide exact order of backups.