Revert "download_strategy: escape spaces in git submodule path."

This is causing submodules to be checked out into a path with a literal
"$path" component:

$ find . | grep "\$path$"
./$path
./Carthage/Checkouts/Commandant/$path
./Carthage/Checkouts/Commandant/Carthage/Checkouts/Quick/$path
./Carthage/Checkouts/PrettyColors/$path
./Carthage/Checkouts/Quick/$path
./Carthage/Checkouts/ReactiveCocoa/$path
./Carthage/Checkouts/ReactiveCocoa/Carthage/Checkouts/Quick/$path
./Carthage/Checkouts/ReactiveTask/$path
./Carthage/Checkouts/ReactiveTask/Carthage/Checkouts/Quick/$path
./Carthage/Checkouts/ReactiveTask/Carthage/Checkouts/ReactiveCocoa/$path
./Carthage/Checkouts/ReactiveTask/Carthage/Checkouts/ReactiveCocoa/Carthage.checkout/Quick/$path

This reverts commit 3a5f099297e9ab3b799af5f7a65edd8d64bc7bae.