Speed up filename filtering.

Before there was a `getcwd` syscall for every filename which was filtered.

Instead this is now cached per-run.

- When all files are identified by filename only: ~45% improvement
- When no files are identified by filename only: ~55% improvement

This makes little difference to overall execution, the bigger win is
eliminating the `memoize_by_cwd` hack.  Just removing the memoization would
have *increased* the runtime by 300-500%.

pre_commit/util.py

@@ -2,7 +2,6 @@ from __future__ import unicode_literals
 
 import contextlib
 import errno
-import functools
 import os.path
 import shutil
 import stat
@@ -31,23 +30,6 @@ def mkdirp(path):
             raise
 
 
-def memoize_by_cwd(func):
-    """Memoize a function call based on os.getcwd()."""
-    @functools.wraps(func)
-    def wrapper(*args):
-        cwd = os.getcwd()
-        key = (cwd,) + args
-        try:
-            return wrapper._cache[key]
-        except KeyError:
-            ret = wrapper._cache[key] = func(*args)
-            return ret
-
-    wrapper._cache = {}
-
-    return wrapper
-
-
 @contextlib.contextmanager
 def clean_path_on_failure(path):
     """Cleans up the directory on an exceptional failure."""