Bug 1341213 - Make the index path for toolchain jobs independent of the source path. r=dustin

When I refactored hash_paths to add caching, I mixed things up such that
for each file, we end up hashing:
  (u'$sha256sum', u'$topsrcdir/$relpath') $relpath

when the intent was to hash:
  $sha256sum $relpath

This change fixes it, such that now the index paths are independent of
the source path, as originally intended.

--HG--
extra : rebase_source : 8ff7b49927d2365ed87fa06d8e6fca157faddc7d

taskcluster/taskgraph/util/hash.py

@@ -12,8 +12,7 @@ import hashlib
 @memoize
 def _hash_path(path):
     with open(path) as fh:
-        return (hashlib.sha256(fh.read()).hexdigest(),
-                mozpath.normsep(path))
+        return hashlib.sha256(fh.read()).hexdigest()
 
 
 def hash_paths(base_path, patterns):