Before this change we calculated all possible hashes for the file when
the `Hashes` method was called.
After we only calculate the Hash requested.
Almost all uses of `Hash` just need one checksum. This will slow down
`rclone lsjson` with the `--hash` flag. Perhaps lsjson should have a
`--hash-type` flag.
However it will speed up sync/copy/move/check/md5sum/sha1sum etc.
Before it took 12.4 seconds to md5sum a 1GB file, after it takes 3.1
seconds which is the same time the md5sum utility takes.
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