Auto merge of #5181 - alexcrichton:selectively-write-dep-info, r=matklad
authorbors <bors@rust-lang.org>
Wed, 14 Mar 2018 22:17:18 +0000 (22:17 +0000)
committerbors <bors@rust-lang.org>
Wed, 14 Mar 2018 22:17:18 +0000 (22:17 +0000)
Don't rewrite dep-info files if they don't change

Similar to how we treat lock files, read the contents, compare, and if they're
the same don't actually write the file.

Closes #5172


Trivial merge