git_config_get_value to return None

- use git_config_get_value instead of "git config"
- return None when no value is specified
- fail with git error code >= 128 if .gitconfig
  has illegal format or some other serious git problem happens.

Change-Id: Icedfb6a281948056f57bf1581ef186b503ba6b94
Allow per-user override of -R setting.

Introduce "gitreview.rebase" user configuration
option.

Change-Id: I96dfab1af48d8f1592766fea4b97e0f35c03d8a5
Switch to flake8 from pep8.

Change-Id: Ib7e1e51e644dbdfc530a7c5ca3005dd73ffa3f79
Align to OpenStack Hacking guidelines.

Change-Id: I8eeea3be46d4943c462772de26b10142f426a00c
Merge "git_config_get_value to return None"
Merge "Allow per-user override of -R setting."
Merge "Switch to flake8 from pep8."
Merge "Align to OpenStack Hacking guidelines."