rebase: remove --allow-empty-message from incompatible opts

--allow-empty-message was turned into a no-op and even documented
as such; the flag is simply ignored.  Since the flag is ignored, it
shouldn't be documented as being incompatible with other flags.

Signed-off-by: Elijah Newren <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: newren

Documentation/git-rebase.txt

@@ -647,7 +647,6 @@ are incompatible with the following options:
  * --merge
  * --strategy
  * --strategy-option
- * --allow-empty-message
  * --[no-]autosquash
  * --rebase-merges
  * --interactive