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 #1123

The iterator used to yield Commit() objects, which does not play well
with inheritance. Yield cls() instead.

Signed-off-by: Yuri Volchkov <yuri@volch.org>

There's no easy way to re-create a commit (i.e. for rewriting purposes), because dates must be formatted as strings, passed, then parsed back.

This patch allows parse_date() to accept datetime instances, such as those produced by from_timestamp() above.

Signed-off-by: Liam Beguin <liambeguin@gmail.com>

Fixes #943