move --from, copy --from: 10 times faster scanning remote on local disk

Rather than go through the location log to see which files are present on
the remote, it simply looks at the disk contents directly.

I benchmarked this speeding up scanning 834 files, from an annex on my
phone's SSD, from 11.39 seconds to 1.31 seconds. (No files actually moved.)

Also benchmarked 8139 files, from an annex on spinning storage,
speeding up from 103.17 to 13.39 seconds.

Note that benchmarking with an encrypted annex on flash actually showed a
minor slowdown with this optimisation -- from 13.93 to 14.50 seconds. Seems
the overhead of doing the crypto needed to get the filenames to directly
check can be higher than the overhead of looking up data in the location
log. (Which says good things about how well the location log and git have
been optimised!) It *may* make sense to make encrypted local remotes not
have hasKeyCheap set; further benchmarking is called for.

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