remote: use remote_is_configured() for add and rename

remote: use remote_is_configured() for add and rename

Both remote add and remote rename use a slightly different hand-rolled
check if the remote exits.  The hand-rolled check may have some subtle
cases in which it might fail to detect when a remote already exists.
One such case was fixed in fb86e32 ("git remote: allow adding remotes
agreeing with url.<...>.insteadOf").  Another case is when a remote is
configured as follows:

  [remote "foo"]
    vcs = bar

If we try to run `git remote add foo bar` with the above remote
configuration, git segfaults.  This change fixes it.

In addition, git remote rename $existing foo with the configuration for
foo as above silently succeeds, even though foo already exists,
modifying its configuration.  With this patch it fails with "remote foo
already exists".

Signed-off-by: Thomas Gummerer <t.gummerer@gmail.com>
Reviewed-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>