Auto merge of #2915 - matklad:rename-shell, r=alexcrichton
Auto merge of #2915 - matklad:rename-shell, r=alexcrichton

Minor, rename function parameter

Naming `Config` as `shell` (while having a `shell` function in scope which returns `MultiShell`) inside this not so trivial callback driven fragment of code is devilishly evil :)

eminence/cargo,rust-lang/cargo,vojtechkral/cargo,rustyhorde/cargo,tsurai/cargo,vojtechkral/cargo,achanda/cargo,AndiDog/cargo,eminence/cargo,brson/cargo,AndiDog/cargo,brson/cargo,AndiDog/cargo,rust-lang/cargo,nipunn1313/cargo,achanda/cargo,vojtechkral/cargo,vojtechkral/cargo,tsurai/cargo,durka/cargo,rust-lang/cargo,rustyhorde/cargo,tshepang/cargo,sbeckeriv/cargo,AndiDog/cargo,rust-lang/cargo,durka/cargo,sbeckeriv/cargo,eminence/cargo,achanda/cargo,AndiDog/cargo,brson/cargo,eminence/cargo,durka/cargo,sbeckeriv/cargo,tshepang/cargo,achanda/cargo,brson/cargo,rustyhorde/cargo,tsurai/cargo,tsurai/cargo,nipunn1313/cargo,tshepang/cargo,tshepang/cargo,nipunn1313/cargo,brson/cargo,nipunn1313/cargo,durka/cargo,tsurai/cargo,rustyhorde/cargo,sbeckeriv/cargo