Summary:
Previously, we would only allow you to consume a symlink that was
checked into a revision, but not allow you to create a new one.
This diff adds support for making new symlinks.
It turns out that the code that we had for readlink had an unused code path for
reading an actual symlink out of the overlay. My first pass at this diff tried
to make the materialization code generate such a symlink, but it was breakin
some other assumptions in the rest of the code.
This version of the code just stores the symlink target in the file contents.
This means that we can simplify readlink to just calling `readAll` on the
underlying file data, and that will load the contents out of the storage layer
if the file wasn't materialized. This helps simplify things a bit more.
Reviewed By: bolinfest, simpkins
Differential Revision: D4604016
fbshipit-source-id: 3138d0f9880639d2bbeaf4bb03bef3f021c3ecb3
