Bug: duplicate sections in .git/config after remote removal

[Jason Frey <jfrey@redhat.com>]

While the impact of this bug is minimal, and git itself is not
affected, it can affect external tools that want to read the
.git/config file, expecting unique section names.

To reproduce:

Given the following example .git/config file (I am leaving out the
[core] section for brevity):

    [remote "origin"]
        url = git@github.com:Fryguy/example.git
        fetch = +refs/heads/*:refs/remotes/origin/*
    [branch "master"]
        remote = origin
        merge = refs/heads/master

Running `git remote rm origin` will result in the following contents:

    [branch "master"]

Running `git remote add origin git@github.com:Fryguy/example.git` will
result in the following contents:

    [branch "master"]
    [remote "origin"]
        url = git@github.com:Fryguy/example.git
        fetch = +refs/heads/*:refs/remotes/origin/*

And finally, running `git fetch origin; git branch -u origin/master`
will result in the following contents:

    [branch "master"]
    [remote "origin"]
        url = git@github.com:Fryguy/example.git
        fetch = +refs/heads/*:refs/remotes/origin/*
    [branch "master"]
        remote = origin
        merge = refs/heads/master

at which point you can see the duplicate sections (even though one is
empty).  Also note that if you do the steps again, you will be left
with 3 sections, 2 of which are empty.  This process can be repeated
over and over.

Thanks,
Jason