shelve: allow multiple shelves with --patch and --stat Before this patch, there was a single way to see multiple shelves using `--patch --list` which show all the shelves. Doing `--patch s1 s2` returns an error. This patch allows to show multiple shelves using `--patch` and `--stat`.

zstd: vendor python-zstandard 0.6.0 Commit 63c68d6f5fc8de4afd9bde81b13b537beb4e47e8 from https://github.com/indygreg/python-zstandard is imported without modifications (other than removing unwanted files). This includes minor performance and feature improvements. It also changes the vendored zstd library from 1.1.1 to 1.1.2. # no-check-commit

util: add length argument to util.buffer() util.buffer() either returns inbuilt buffer function or defines a new one which slices. The inbuilt buffer() also has a length argument which is missing from the ones we defined. This patch adds that length argument.

py3: replace pycompat.getenv with encoding.environ.get pycompat.getenv returns os.getenvb on py3 which is not available on Windows. This patch replaces them with encoding.environ.get and checks to ensure no new instances of os.getenv or os.setenv are introduced.