filectx: replace use of _filerev with _filenode _filerev depends on the filelog implementation using revlogs and linkrevs. Alternative implementations, like remotefilelog, do not have rev numbers, so this call fails. Replacing it with _filenode means it doesn't rely on rev numbers, and doesn't cost anything extra, since _filerev is using _filenode under the hood anyway.

verify: extract "manifest" constant into variable The "manifest" label that's used in error messages will instead be the directory path for subdirectory manifests (not the root manifest), so let's extract the constant to a variable already to make future patches simpler.

verify: use similar language for missing manifest and file revisions When a changeset refers to a manifest revision that's not found in the manifest log, we say "changeset refers to missing revision X", but when a manifest refers to file revision that's not found in the filelog, we say "X in manifests not found". The language used for missing manifest revisions seems clearer, so let's use that for missing filelog revisions too.