casecollisionauditor: ignore eden_dirstate_map

Summary:
The default casecollisionauditor reads all of dirstate._map by iterating through it. This is expensive for a large repo and eden now raises RuntimeError for the iterator method. Please check D3964461 (73f0e3d) for more info.

As Jun pointed out:

* For non-edenfs we would always use treestate, therefore the original `if not dirstate._istreestate and not dirstate._istreedirstate:` check always evaluate to `False`.
* For EdenFS, due to above reason, we should not go into the `then`-part of the if statement.

So we can just delete the `then`-part of the if statement:

```
if not dirstate._istreestate and not dirstate._istreedirstate:
    allfiles = "\0".join(dirstate._map)
    self._loweredfiles = set(encoding.lower(allfiles).split("\0"))
```

Reviewed By: quark-zju

Differential Revision: D42210487

fbshipit-source-id: 6c5d5daca9dd85deae6505344a4abad2cafb2831

Author: zzl0

eden/scm/edenscm/scmutil.py

@@ -442,13 +442,9 @@ class casecollisionauditor(object):
     def __init__(self, ui, abort, dirstate):
         self._ui = ui
         self._abort = abort
-        if not dirstate._istreestate and not dirstate._istreedirstate:
-            allfiles = "\0".join(dirstate._map)
-            self._loweredfiles = set(encoding.lower(allfiles).split("\0"))
-        else:
-            # Still need an in-memory set to collect files being tested, but
-            # haven't been added to treestate yet.
-            self._loweredfiles = set()
+        # Still need an in-memory set to collect files being tested, but
+        # haven't been added to treestate yet.
+        self._loweredfiles = set()
         self._dirstate = dirstate
         # The purpose of _newfiles is so that we don't complain about
         # case collisions if someone were to call this object with the