remote: allow adding remote w same name as alias

Then ~/.gitconfig contains an alias (i.e. myremote)
and you are adding a new remote using the same name
for remote, Git will refuse to add the remote with
the same name as one of the aliases, even though the
remote with such name is not setup for current repo.

$ git remote add myremote git@host.com:team/repo.git
fatal: remote myremote already exists.

The fatal error comes from strcmp(name, remote->url[0])
condition, which compares a name of the new remote with
existing urls of all the remotes, including the ones
from ~/.gitconfig (or global variant).
I'm not sure why that is necessary, unless Git is meant
to prevent users from naming their remotes as their
remote aliases..
Imho, if someone want's to git remote add myremote
myremote, they should, since git-remote always takes
2 arguments, first being the new remote's name and
second being new remote's url (or alias, if set).
Thanks to @mymuss for sanity check and debugging.