clone: forbid --bare --separate-git-dir <dir>

The --separate-git-dir option was introduced to make it simple to put
the git directory somewhere outside the worktree, for example when
cloning a repository for use as a submodule.

It was not intended for use when creating a bare repository. In that
case there is no worktree and it is more natural to directly clone the
repository and create a .git file as separate steps:

        git clone --bare /path/to/repo.git bar.git
        printf 'gitdir: bar.git\n' >foo.git

Forbid the combination, making the command easier to explain.
Signed-off-by: NNguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

builtin/clone.c

@@ -704,6 +704,8 @@ int cmd_clone(int argc, const char **argv, const char *prefix)
 		if (option_origin)
 			die(_("--bare and --origin %s options are incompatible."),
 			    option_origin);
+		if (real_git_dir)
+			die(_("--bare and --separate-git-dir are incompatible."));
 		option_no_checkout = 1;
 	}