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>

In c96476b, the new default_name nested function does not contain a
retun statement. This leads to an issue when the environment variables
are not present, where the actor name would not be set.

Signed-off-by: Athos Ribeiro <athos@redhat.com>

Calling getpass.getuser may lead to breakage in environments where there
is no entries in the /etc/passwd file for the current user.

Setting the environment variables for the git user configurations should
prevents GitPython from using values from /etc/passwd. However, doing so
will not prevent reading /etc/passwd and looking for an entry with the
current user UID.

This patch changes the behavior described above so GitPython will
perform a lazy evaluation of /etc/passwd, only doing so when the
environment variables for the git user configuration are not available.

Signed-off-by: Athos Ribeiro <athos@redhat.com>