Auto merge of #4065 - michaelwoerister:smarter-cargo-incremental, r=alexcrichton
Auto merge of #4065 - michaelwoerister:smarter-cargo-incremental, r=alexcrichton

Only compile user-modifiable projects incrementally.

Fixes #3972. Thanks for pointing me to the correct implementation, Alex!

r? @alexcrichton

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