git-gui: Allow users to edit user.name, user.email from options.

git-gui: Allow users to edit user.name, user.email from options.

Users may need to be able to alter their user.name or user.email
configuration settings.  If they are mostly a git-gui user they
should be able to view/set these important values from within
the git-gui environment, rather than needing to edit a raw text
file on their local filesystem.

Signed-off-by: Shawn O. Pearce <redacted>