scmutil: teach the file prefetch hook to handle multiple commits The remainder of the commands that need prefetch deal with multiple revisions. I initially coded this as a separate hook, but then it needed a list of files to handle `diff` and `grep`, so it didn't seem worth keeping them separate. Not every matcher will emit bad file messages (some are built from a list of files that are known to exist). But it seems better to filter this in one place than to push this on either each caller or each hook implementation.