backups: add more info, keep 1 of each version opt

backup's filename format has changed, and includes the commit too now. a
Backup struct for going to/from the filename has been added, and the
option "keep 1 backup from each version" has been added, leaving the
most recent backup from each version always. All pre-this-commit backups
are considered the same "old" version.

logmessages/logmessages.go

@@ -200,6 +200,7 @@ const (
 	DeleteOldBackup       = "Deleted old backup \"%s\""
 	FailedDeleteOldBackup = "Failed to delete old backup \"%s\": %v"
 	CreateBackup          = "Created database backup \"%+v\""
+	FailedParseBackup     = "Failed to parse backup \"%s\": %v"
 	FailedCreateBackup    = "Faled to create database backup: %v"
 	MoveOldDB             = "Moved existing database to \"%s\""
 	FailedMoveOldDB       = "Failed to move existing database to \"%s\": %v"