Cap timestamps in index to avoid needing to worry about fractional parts.

Cap timestamps in index to avoid needing to worry about fractional parts.

Avoid a potential race condition by which bup's use of whole-second
granularity for timestamps in the index could let it theoretically
miss some last-second changes by capping timestamps to at most one
second before the start of indexing per a newly introduced mandatory
parameter to bup.index.Writer.