cache dependency mtimes and check equality, not mtime(dep) <= mtime(cache)

[stevengj]

As discussed in #12458, it is a good idea to store the timestamp of the dependency files in the .ji file, so that we can check staleness by comparing that the dependency timestamp is equal to the cached value, not just that the dependency is older than the cachefile (ala make). This has two advantages:

• Correctness: if you replace a dependency with an older version of the file, you still want to recompile.
• Clock-skew: this should greatly reduce the problems arising from clock-skew if you are manually copying files back and forth over the network (cc @tkelman).

Of course, we may still want to use a hash or checksum at some point, but this is a free-of-cost thing that we can do now.

[pao]

Seems perfectly cromulent.

[ScottPJones]

👍 (still wanting to see simple hashing at some point though 😀)

[tkelman]

Cool, I think this'll help. Will test it under the setup where I was hitting trouble and let you know.

[tkelman]

So far so good in initial tests.