Start up compat and encoding files

There were a few things which were being reused consistently for
compatibility purposes, such as the `buffer`/`memoryview` functions
as well as the `izip` method which needed to be aliased for
Python 3.  The `buffer` function was taken from `smmap` [1] and
reworked slightly to handle the optional third parameter.

This also adds a compatibility file dedicated entirely to encoding
issues, which seem to be the biggest problem.  The main functions
were taken in part from the Django project [2] and rewritten
slightly because our needs are a bit more narrow.

A constants file has been added to consistently handle the
constants which are required for the gitdb project in the core
and the tests.  This is part of a greater plan to reorganize
the `util.py` file included in this project.

This points the async extension back at the original repository
and points it to the latest commit.

[1]: https://github.com/Byron/smmap/blob/1af4b42a2354acbb53c7956d647655922658fd80/smmap/util.py#L20-L26
[2]: https://github.com/django/django/blob/b8d255071ead897cf68120cd2fae7c91326ca2cc/django/utils/encoding.py

1 files changed, 3 insertions, 7 deletions

diff --git a/gitdb/util.py b/gitdb/util.py
index 30c7008..5a82c55 100644
--- a/gitdb/util.py
+++ b/gitdb/util.py
@@ -30,10 +30,7 @@ else:
     mman = SlidingWindowMapManager()
 #END handle mman
 
-try:
-    import hashlib
-except ImportError:
-    import sha
+import hashlib
 
 try:
     from struct import unpack_from
@@ -84,9 +81,8 @@ write = os.write
 close = os.close
 fsync = os.fsync
 
-# constants
-NULL_HEX_SHA = "0"*40
-NULL_BIN_SHA = "\0"*20
+# Backwards compatibility imports
+from gitdb.const import NULL_BIN_SHA, NULL_HEX_SHA
 
 #} END Aliases