I believe apm must have been buggy - example output that I found online
showed `tr` was mangling paths with spaces in it. Should be fixed.
Also, use dscl on OS X in __fish_complete_users.fish like
__fish_print_users.fish already does.
This is already done by fish before calling the completion.
It breaks completion with combiners (#2025) and also with wrappers.
(This does not include git because that's better solved in #2145)