remote: actually check if remote exits

remote: actually check if remote exits

When converting the git remote command to a builtin in 211c89 ("Make
git-remote a builtin"), a few calls to check if a remote exists were
converted from:
       if (!exists $remote->{$name}) {
       	  [...]
to:
       remote = remote_get(argv[1]);
       if (!remote)
          [...]

The new check is not quite correct, because remote_get() never returns
NULL if a name is given.  This leaves us with the somewhat cryptic error
message "error: Could not remove config section 'remote.test'", if we
are trying to remove a remote that does not exist, or a similar error if
we try to rename a remote.

Use the remote_is_configured() function to check whether the remote
actually exists, and die with a more sensible error message ("No such
remote: $remotename") instead if it doesn't.

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>