[BUG] Possible bug in `remote set-url --add --push`

Hi,

I believe `remote set-url --add --push` has a bug. Performed tests
with v1.8.0.1 and v1.8.1 (Mac OS X).

Quoting the relevant part of the documentation:

set-url
    Changes URL remote points to. Sets first URL remote points to matching regex <oldurl> (first URL if no <oldurl> is given) to <newurl>. If <oldurl> doesn’t match any URL, error occurs and nothing is changed.

    With --push, push URLs are manipulated instead of fetch URLs.
    With --add, instead of changing some URL, new URL is added.
    With --delete, instead of changing some URL, all URLs matching regex <url> are deleted. Trying to delete all non-push URLs is an error.

Here are some steps to reproduce:

1. Show the remote URLs

jweyrich@pharao:test_clone1 [* master]$ git remote -v
origin  /Volumes/sandbox/test (fetch)
origin  /Volumes/sandbox/test (push)

2. Add a new push URL for origin

jweyrich@pharao:test_clone1 [* master]$ git remote set-url --add --push origin \
    /Volumes/sandbox/test_clone2

3. Check what happened

jweyrich@pharao:test_clone1 [* master]$ git remote -v
origin  /Volumes/sandbox/test (fetch)
origin  /Volumes/sandbox/test_clone2 (push)

4. Missing an URL? Re-add the original one

jweyrich@pharao:test_clone1 [* master]$ git remote set-url --add --push origin \
    /Volumes/sandbox/test

5. Check what happened, again

jweyrich@pharao:test_clone1 [* master]$ git remote -v
origin  /Volumes/sandbox/test (fetch)
origin  /Volumes/sandbox/test_clone2 (push)
origin  /Volumes/sandbox/test (push)

In step 2, Git replaced the original push URL instead of adding a new
one. But it seems to happen only the first time I use `remote set-url
--add --push`. Re-adding the original URL using the same command seems
to work properly.
And FWIW, if I delete (with "set-url --delete") both URLs push, Git
restores the original URL.

Please, could someone try to reproduce?

- jw