Allow equal time-stamps to be considered up-to-date

[smyrman]

Proposal

A task should be considered up-to-date if the newest timestamp for sources is exactly equal to the oldest timestamp from generates.

Background

On Mac, the default filesystem, HFS, only supports second precession for file modification timestamps. Windows NTFS (in theory) supports 100-nanosecond interval precession, while ext4 on Linux rely on cached timestamps from the kernel which in most cases would at least be about 100 ms apart.

Especially on HFS, this can create some hard-to-debug issues where tasks some times are considered up-to-date, and sometimes not.

While writing the tests for #47, it took me a while to figure out why my test worked (sometimes) when I run task manually in "testdata/generates", but never when I run it from task_test.go. For real-life use-cases, this would help with getting a more consistent behavior when we have task-output from one task used as input in the next (short-running) task.