rebase: fix an incompatible-options error message

When the user specifies the apply backend with options that only work
with the merge backend, such as

    git rebase --apply --exec /bin/true HEAD~3

the error message has always been

    fatal: --exec requires an interactive rebase

This error message is misleading and was one of the reasons we renamed
the interactive backend to the merge backend.  Update the error message
to state that these options merely require use of the merge backend.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/rebase.c

@@ -561,7 +561,7 @@ static void imply_merge(struct rebase_options *opts, const char *option)
 {
 	switch (opts->type) {
 	case REBASE_APPLY:
-		die(_("%s requires an interactive rebase"), option);
+		die(_("%s requires the merge backend"), option);
 		break;
 	case REBASE_MERGE:
 	case REBASE_PRESERVE_MERGES: