Support Git commits with no angle brackets in author name

$ git clone https://github.com/noderabbit-team/tasks
$ python3
>>> import dulwich.repo
>>> r = dulwich.repo.Repo('tasks')
>>> c = dulwich.repo.Repo('.')[b'c8a1c472c97789b44d91256d5cb8899a461a0b22')
>>> c.as_raw_string()
b'tree 3e8bd4afdc836d866ad57089ad565b5f09e38475\nparent 361db946b8e11475869a125768185fdf5b77e8b3\nauthor kapil.foss@gmail.com 1297013737 -0500\ncommitter kapil.foss@gmail.com 1297013737 -0500\nencoding utf-8\n\nmore pep8, verify archive file exists before removing, test docstring'
>>> c.author
b'kapil.foss@gmail.com 1297013737 -0500'
>>> c.author_time

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Migrated from T3880 (view on Phabricator)