rebase: help users when dying with preserve-merges

Git will die if a "rebase --preserve-merges" is in progress.
Users cannot --quit, --abort or --continue the rebase.

Make the `rebase --abort` option available to allow users to remove
traces of any preserve-merges rebase, even if they had upgraded
during a rebase.

One trigger was an unexpectedly difficult to resolve conflict, as
reported on the `git-users` group.
(https://groups.google.com/g/git-for-windows/c/3jMWbBlXXHM)

Tell the user the options to resolve the problem manually.

Signed-off-by: Philip Oakley <[email protected]>

Author: PhilipOakley

builtin/rebase.c

@@ -1182,8 +1182,10 @@ int cmd_rebase(int argc, const char **argv, const char *prefix)
 	} else if (is_directory(merge_dir())) {
 		strbuf_reset(&buf);
 		strbuf_addf(&buf, "%s/rewritten", merge_dir());
-		if (is_directory(buf.buf)) {
-			die("`rebase -p` is no longer supported");
+		if (is_directory(buf.buf) && !(action == ACTION_ABORT)) {
+			die("`rebase --preserve-merges` (-p) is no longer supported.\n"
+			"Use `git rebase --abort` to terminate current rebase.\n"
+			"Or downgrade to v2.33, or earlier, to complete the rebase.\n");
 		} else {
 			strbuf_reset(&buf);
 			strbuf_addf(&buf, "%s/interactive", merge_dir());