Auto merge of #1261 - alexcrichton:issue-1259, r=wycats

Previously if a fingerprint was considered fresh based on mtime, it would not
get updated once a compilation finished with the new precise fingerprint (it
would use the older fingerprint). This alters the `resolve` method to take a
flag which disables this behavior and forces looking at the filesystem for a
fingerprint.

Closes #1259