convert: use None value for missing files instead of overloading IOError The internal API used IOError to indicate that a file should be marked as removed. There is some correlation between IOError (especially with ENOENT) and files that should be removed, but using IOErrors to represent file removal internally required some hacks. Instead, use the value None to indicate that the file not is present. Before, spurious IO errors could cause commits that silently removed files. They will now be reported like all other IO errors so the root cause can be fixed.