shelve: trust caller of shelvedfile.opener() to check that the file exists The only place we call `shelvedfile.opener()` is when we're about to apply a bundle. The file should always exist. If it doesn't, the `.hg/` directory is corrupt and we don't provide any guarantees about supporting corrupt repos (besides, telling the user that the shelve doesn't exist when `hg shelve --list` lists it is not very helpful). Differential Revision: https://phab.mercurial-scm.org/D9698

To run the tests, do:

cd tests/
python run-tests.py

See https://mercurial-scm.org/wiki/WritingTests for
more information on writing tests.