largefiles: adapt remotestore._getfile to batched statlfile
9e1616307c4c introduced batching of statlfile, but not all codepaths got
converted.
_getfile gave _stat garbage and got garbage back. The garbage didn't match the
expected error codes and was thus interpreted as success. It could thus end up
trying to fetch a largefile that didn't exist.
Instead we now pass _stat valid input and handle both correct and invalid
output correctly.
This makes the code work as intended ... but it would probably be better if it
didn't abort on missing largefiles, just like it happened to do before.