rebase: emit one "fatal" in "fatal: fatal: <error>"

The die() routine adds a "fatal: " prefix, there is no reason to add
another one. Fixes code added in e65123a71d (builtin rebase: support
`git rebase <upstream> <switch-to>`, 2018-09-04).

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/rebase.c

@@ -1918,7 +1918,7 @@ int cmd_rebase(int argc, const char **argv, const char *prefix)
 						   &options.orig_head))
 			options.head_name = NULL;
 		else
-			die(_("fatal: no such branch/commit '%s'"),
+			die(_("no such branch/commit '%s'"),
 			    branch_name);
 	} else if (argc == 0) {
 		/* Do not need to switch branches, we are already on it. */