export: invoke the file prefetch hook cmdutil.exportfile() is only called by shelve, mq and patchbomb. Those are unlikely to mix with lfs, but it may as well be invoked there for completeness.

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.

tests: arrange for a server in wireproto-command-capabilities.t to be killed The stray servers were piling up after the test harness exited. On Windows, this means the *.pyd files can't be rebuilt, which is why the build warning count dropped to 1 recently.