git-gui: Implement system-wide configuration handling.

git-gui: Implement system-wide configuration handling.

With the old implementation any system-wide options appear
to be set locally in the current repository. This commit
adds explicit handling of system options, essentially
interpreting them as customized default_config.

The difficulty in interpreting system options stems from
the fact that simple 'git config' lists all values, while
'git config --global' only values set in ~/.gitconfig,
excluding both local and system options.

Signed-off-by: Alexander Gavrilov <angavrilov@gmail.com>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>