bundle: let "-" mean stdin for reading operations

For writing, "bundle create -" indicates that the bundle should be written to stdout. But there's no matching handling of "-" for reading operations. This is inconsistent, and a little inflexible (though one can always use "/dev/stdin" on systems that support it). However, it's easy to change. Once upon a time, the bundle-reading code required a seekable descriptor, but that was fixed long ago in e9ee84cf28b (bundle: allowing to read from an unseekable fd, 2011-10-13). So we just need to handle "-" explicitly when opening the file. We _could_ do this by handling "-" in read_bundle_header(), which the reading functions all call already. But that is probably a bad idea. It's also used by low-level code like the transport functions, and we may want to be more careful there. We do not know that stdin is even available to us, and certainly we would not want to get confused by a configured URL that happens to point to "-". So instead, let's add a helper to builtin/bundle.c. Since both the bundle code and some of the callers refer to the bundle by name for error messages, let's use the string "<stdin>" to make the output a bit nicer to read. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>