i18n: Rewrite gettext messages start with dash

Gettext message in a shell script should not start with '-', one
workaround is adding '--' between gettext and the message, like:

    gettext -- "--exec option ..."

But due to a bug in the xgettext extraction, xgettext can not
extract the actual message for this case. Rewriting the message
is a simpler and better solution.

Reported-by: Vincent van Ravesteijn <vfr@lyx.org>
Signed-off-by: Jiang Xin <worldhello.net@gmail.com>
Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Reviewed-by: Stefano Lattarini <stefano.lattarini@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-rebase.sh

@@ -317,7 +317,7 @@ test $# -gt 2 && usage
 if test -n "$cmd" &&
    test "$interactive_rebase" != explicit
 then
-	die "$(gettext -- "--exec option must be used with --interactive option")"
+	die "$(gettext "The --exec option must be used with the --interactive option")"
 fi
 
 if test -n "$action"