largefiles: fix addremove with -R option If a file was missing, the missing list contained a path relative to the repo. When building the matcher from that list, the file name ended up concatenated to cwd, causing the command to abort with '<file> not under root'. This rebuilds the missing list with paths relative to cwd.

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