git-remote: reject adding remotes with invalid names

git-remote: reject adding remotes with invalid names

This can happen if the arguments to git-remote add is switched by the
user, and git would only show an error if fetching was also requested.
Fix it by using the refspec parsing engine to check if the requested
name can be parsed as a remote before add it.

Also cleanup so that the "remote.<name>.url" config name buffer is only
initialized once.

Signed-off-by: Jonas Fonseca <fonseca@diku.dk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>