Load the whole config at once and cache it for future use

Load the whole config at once and cache it for future use

Instead of loading the config values one at at time when we need then,
read and parse the configuration files all at once using

  git config --list --null

and cache the result. This should be a performance win.

Signed-off-by: Samuel Tardieu <sam@rfc1149.net>
Signed-off-by: Karl Hasselström <kha@treskal.com>