Add setter methods & write support for git_commit

Add setter methods & write support for git_commit

All the required git_commit_set_XXX methods have been implemented; all
the attributes of a commit object can now be modified in-memory.

The new method git_object_write() automatically writes back the
in-memory changes of any object to the repository. So far it only
supports git_commit objects.

Signed-off-by: Vicent Marti <tanoku@gmail.com>