Auto merge of #3259 - alexcrichton:more-ro, r=brson
authorbors <bors@rust-lang.org>
Tue, 8 Nov 2016 14:47:48 +0000 (06:47 -0800)
committerGitHub <noreply@github.com>
Tue, 8 Nov 2016 14:47:48 +0000 (06:47 -0800)
Open crate files readonly first

This allows Cargo to work with read-only `CARGO_HOME` directories where the
cache was prepared ahead of time.

Closes #3256


Trivial merge