rebase: add coverage of other incompatible options

The git-rebase manual noted several sets of incompatible options, but
we were missing tests for a few of these.  Further, we were missing
code checks for one of these, which could result in command line
options being silently ignored.

Also, note that adding a check for autosquash means that using
--whitespace=fix together with the config setting rebase.autosquash=true
will trigger an error.  A subsequent commit will improve the error
message.

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

builtin/rebase.c

@@ -1511,6 +1511,9 @@ int cmd_rebase(int argc, const char **argv, const char *prefix)
 	if (options.update_refs)
 		imply_merge(&options, "--update-refs");
 
+	if (options.autosquash)
+		imply_merge(&options, "--autosquash");
+
 	if (options.type == REBASE_UNSPECIFIED) {
 		if (!strcmp(options.default_backend, "merge"))
 			imply_merge(&options, "--merge");