Ask git for author and committer name

(patch modified by Catalin Marinas)

Consistently do the following to get hold of default user, author and
committer:

  1. Use the value specified on the command line, if any.

  2. Otherwise, use the environment variables, if any.

  3. Otherwise, read the configuration files

Signed-off-by: Karl Hasselström <kha@treskal.com>
Signed-off-by: Catalin Marinas <catalin.marinas@gmail.com>