merge: clarify call chain

prepare_to_commit() cannot be reached in the non-squash case:
It is called by merge_trivial() and finish_automerge() only, but the
calls to the latter are somewhat hard to track:

If option_commit is not set, the code in cmd_merge() uses a fake
conflict return code (ret=1) to avoid writing the tree, which also
avoids setting automerge_was_ok (just as in the proper ret==1 case), so
that finish_automerge() is not called.

To ensure that no code change breaks that assumption, safe-guard
prepare_to_commit() by a BUG() statement.
Suggested-by: NJunio C Hamano <gitster@pobox.com>
Signed-off-by: NMichael J Gruber <git@grubix.eu>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

builtin/merge.c

@@ -761,6 +761,8 @@ static void prepare_to_commit(struct commit_list *remoteheads)
 	struct strbuf msg = STRBUF_INIT;
 	strbuf_addbuf(&msg, &merge_msg);
 	strbuf_addch(&msg, '\n');
+	if (squash)
+		BUG("the control must not reach here under --squash");
 	if (0 < option_edit)
 		strbuf_commented_addf(&msg, _(merge_editor_comment), comment_line_char);
 	write_file_buf(git_path_merge_msg(), msg.buf, msg.len);