link_alt_odb_entry: handle normalize_path errors

When we add a new alternate to the list, we try to normalize out any redundant "..", etc. However, we do not look at the return value of normalize_path_copy(), and will happily continue with a path that could not be normalized. Worse, the normalizing process is done in-place, so we are left with whatever half-finished working state the normalizing function was in. Fortunately, this cannot cause us to read past the end of our buffer, as that working state will always leave the NUL from the original path in place. And we do tend to notice problems when we check is_directory() on the path. But you can see the nonsense that we feed to is_directory with an entry like: this/../../is/../../way/../../too/../../deep/../../to/../../resolve in your objects/info/alternates, which yields: error: object directory /to/e/deep/too/way//ects/this/../../is/../../way/../../too/../../deep/../../to/../../resolve does not exist; check .git/objects/info/alternates. We can easily fix this just by checking the return value. But that makes it hard to generate a good error message, since we're normalizing in-place and our input value has been overwritten by cruft. Instead, let's provide a strbuf helper that does an in-place normalize, but restores the original contents on error. This uses a second buffer under the hood, which is slightly less efficient, but this is not a performance-critical code path. The strbuf helper can also properly set the "len" parameter of the strbuf before returning. Just doing: normalize_path_copy(buf.buf, buf.buf); will shorten the string, but leave buf.len at the original length. That may be confusing to later code which uses the strbuf. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>