Minor bug: git config ignores empty sections

Looks like there's a problem with setting a config with an empty section, making it create a new section. The result is:

    $ git --version
    git version 2.9.2
    $ git init
    Initialized empty Git repository in /home/eli/t/.git/
    $ t() { git config x.y x; git config --unset x.y; }
    $ t;t;t
    $ grep -c '\[x\]' .git/config
    3
    $ git config x.z x
    $ t;t;t
    $ git config x.z x     # adds another [x], but leaves it populated
    $ t;t;t;t;t;t;t;t
    $ grep -c '\[x\]' .git/config
    4