help_unknown_cmd: do not propose an "unknown" cmd

help_unknown_cmd: do not propose an "unknown" cmd

When executing an external shell script like `git foo` with a bad
shebang, e.g. "#!/usr/bin/not/existing", execvp returns 127 (ENOENT).

Since help_unknown_cmd proposes the use of all external commands similar
to the name of the "unknown" command, it suggests the just failed command
again. Stop it and give some advice to the user.

Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Michael Schubert <mschub@elegosoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>