[WIP] add replace method to git.Commit

(marked WIP until I add appropriate tests)

This adds a replace method to git.Commit. The replace method returns a
copy of the Commit object with attributes replaced from keyword
arguments. For example:

>>> old = repo.head.commit
>>> new = old.replace(message='This is a test')

closes gitpython-developers#1123

Author: larsks

git/objects/commit.py

@@ -136,6 +136,24 @@ def __init__(self, repo, binsha, tree=None, author=None, authored_date=None, aut
     def _get_intermediate_items(cls, commit):
         return commit.parents
 
+    @classmethod
+    def _calculate_sha_(cls, repo, commit):
+        stream = BytesIO()
+        commit._serialize(stream)
+        streamlen = stream.tell()
+        stream.seek(0)
+
+        istream = repo.odb.store(IStream(cls.type, streamlen, stream))
+        return istream.binsha
+
+    def replace(self, **kwargs):
+        attrs = {k: getattr(self, k) for k in self.__slots__}
+        attrs.update(kwargs)
+        new_commit = self.__class__(self.repo, self.__class__.NULL_BIN_SHA, **attrs)
+        new_commit.binsha = self._calculate_sha_(self.repo, new_commit)
+
+        return new_commit
+
     def _set_cache_(self, attr):
         if attr in Commit.__slots__:
             # read the data in a chunk, its faster - then provide a file wrapper
@@ -375,13 +393,7 @@ def create_from_tree(cls, repo, tree, message, parent_commits=None, head=False,
                          committer, committer_time, committer_offset,
                          message, parent_commits, conf_encoding)
 
-        stream = BytesIO()
-        new_commit._serialize(stream)
-        streamlen = stream.tell()
-        stream.seek(0)
-
-        istream = repo.odb.store(IStream(cls.type, streamlen, stream))
-        new_commit.binsha = istream.binsha
+        new_commit.binsha = cls._calculate_sha_(repo, new_commit)
 
         if head:
             # need late import here, importing git at the very beginning throws