Assumptions about order of ReadDir results in tail/tail.go

[jmacd]

Reporting this bug discovered in an downstream fork of this repository, here:

lightstep/opentelemetry-prometheus-sidecar#69

This can result in the sidecar exiting with a bogus corruption message when in fact nothing is wrong.

The fix is simple, see the associated PR. If it pleases a maintainer of this code, consider looking at other downstream changes in tail/tail.go that are relevant here (see listSegments()).