log: make -frREV PATH detect missing files before falling back to slow path
If -rREV isn't specified, "log --follow" would abort on nonexistent paths.
Let's implement this behavior for "-frREV" case as we have ctx.hasdir() now.
Otherwise "log -frREV PATH" would silently fall back to slow path and files
wouldn't be followed across renames.
The loop is quadratic (as before), but the size of the startctxs and
match.files() should be small in general.
Some tests are marked as BROKEN since file renames aren't tracked in the
slow path. This is a known limitation of the current history traversal