largefiles: fix cat of non-largefiles from subdirectory
We were calling back to the original commands.cat from inside the walk loop
that handled and filtered out largefiles. That did however happen with file
paths relative to repo root and the original cat would fail when it applied its
own walk and match on top of that.
Instead we now duplicate and modify the code from commands.cat and patch it to
handle both normal and largefiles.
A change in test output shows that this also makes the exit code with
largefiles consistent with the normal one in the case where one of several
specified files are missing.
This also fixes the combination of --output and largefiles.