Add support for long paths

* Add more file i/o wrappers in platform_utils to allow using
  long paths (length > MAX_PATH) on Windows.

* Paths using the long path syntax ("\\?\" prefix) should never
  escape the platform_utils API surface area, so that this
  specific syntax is not visible to the rest of the repo code base.

* Forward many calls from os.xxx to platform_utils.xxx in various place
  to ensure long paths support, specifically when repo decides to delete
  obsolete directories.

* There are more places that need to be converted to support long paths,
  this commit is an initial effort to unblock a few common use cases.

* Also, fix remove function to handle directory symlinks

Change-Id: If82ccc408e516e96ff7260be25f8fd2fe3f9571a

git_refs.py

@@ -15,6 +15,7 @@
 
 import os
 from trace import Trace
+import platform_utils
 
 HEAD      = 'HEAD'
 R_CHANGES = 'refs/changes/'
@@ -127,9 +128,9 @@ class GitRefs(object):
 
   def _ReadLoose(self, prefix):
     base = os.path.join(self._gitdir, prefix)
-    for name in os.listdir(base):
+    for name in platform_utils.listdir(base):
       p = os.path.join(base, name)
-      if os.path.isdir(p):
+      if platform_utils.isdir(p):
         self._mtime[prefix] = os.path.getmtime(base)
         self._ReadLoose(prefix + name + '/')
       elif name.endswith('.lock'):