backticks: empty backticks create Cmd with zero words [#3278].

You can't run a zero-word command, but it's handy for constructing
commands from subcommands using backtick syntax.
remove set operator puns, add issubset(). fixes #3272
Merge branch 'master' of github.com:JuliaLang/julia
Change ispath to isfile in loading.jl to prevent directories breaking file load