Distinguish files with identical contents

[liam923]

Currently Merlin's locate query uses a digest of the contents of a file to disambiguate files with the same name. If two files have the same digest, one is arbitrarily chosen. This can lead to Merlin confusing two files with identical contents. (See test case demonstrating the issue in the first commit.)

This PR resolves this issue by using a heuristic when multiple digests match. In a nutshell, it chooses the file whose path is most similar to the path of the build artifact it is trying to be matched to. See the comment in locate.ml for more detail.

[voodoos]

Thanks Liam, I will have a look.

[voodoos]

Thanks Liam, looks like a nice improvement for that corner case.
I am a bit worried about the leaking exception but apart from that the changes looks good to me.

[voodoos]

Thanks !