Add INDEX and DiffConstants to git.__all__

This adds the names of git.diff.INDEX and git.diff.DiffConstants to
the top-level __all__.

In particular, the recently introduced INDEX constant has the same
standing as NULL_TREE, which was (rightly) already listed, and this
change makes true the claims in some docstrings in the git.diff
module that say that INDEX is accessible (among other ways) as
git.INDEX.

Author: EliahKagan

git/__init__.py

@@ -20,6 +20,7 @@
     "CommandError",
     "Commit",
     "Diff",
+    "DiffConstants",
     "DiffIndex",
     "Diffable",
     "FetchInfo",
@@ -33,6 +34,7 @@
     "HEAD",
     "Head",
     "HookExecutionError",
+    "INDEX",
     "IndexEntry",
     "IndexFile",
     "IndexObject",