largefiles: respect the rev when reading standins in copytostore() (issue3630)
When the rev isn't specified, the standin for the working copy gets read. But
convert doesn't update the working copy for each cset it processes, so there is
no standin and the 'hg convert' would abort complaining about the standin being
missing.
Note that if the largefile is not in the user cache, 'hg convert' complains
about the largefile itself missing from the destination repo.
Greg Ward, author of the original bfiles extension
Na'Tosha Bard of Unity Technologies
Fog Creek Software
Special thanks to the University of Toronto and the UCOSP program