Add a way to link by git commit id

[brson]

It would be great if extern mod could just say exactly what revision of crates it wants, either by branches or tags or hashes.

[pnkfelix]

Not critical for 0.6; de-milestoning

[sanxiyn]

Tagging this A-pkg.

[graydon]

Subsumed in #5684 , closing this