pkg2: formatting (convert tabs to spaces).
edit: call shell split for unknown EDITOR values [fix #3165]
make Dict rehash more robust. fixes #3181

the strategy is to leave the Dict alone and rehash into fresh arrays,
then mutate the Dict atomically at the end.
Merge branch 'master' of github.com:JuliaLang/julia
fix #3177. was due to a missing gc root.