Auto merge of #2186 - alexcrichton:transaction, r=brson

Right now a `Bomb` struct is used to attempt to ensure that broken tarballs
don't escape, but this unfortunately doesn't work for when Cargo is terminated
via other means such as ctrl-c or abnormal termination. Instead the tarball is
constructed in a temporary location and then only moved to the final location
once all checks pass.

Closes #2173
cc #2177