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.
\{
"node": {node|json},
"changeset_count": {changesets|json},
"changesets": [{join(nodes%graphentry, ", ")}]
}