auto merge of #139 : tomjakubowski/cargo/fix-symlink-blowup, r=alexcrichton

Fingerprinting will fail at an `fs::stat()` call if there is a symlink in
a package's directory pointing to a non-existent file or directory.
This commit recovers from an `fs::stat(`) error on these bogus symlinks by
faking an mtime of 0, which should not affect the overall fingerprint.

Fix #135

diff --cc tests/test_cargo_compile.rs
Simple merge