After each variable gets set, we store it in our list (not completely
in the right position, but the close enough). Then we write out the
new config file in the same way that git.git does it (keep the rest of
the file intact and insert or replace the variable in its line).

Overwriting variables and adding new ones is supported (even on new
sections), though deleting isn't yet.

Signed-off-by: Carlos Martín Nieto <cmn@elego.de>