tests/sitecustomize.py

unshelve: don't commit unknown files during unshelve (issue4113)

Previously, unshelve would temporarily commit unknown files (via addremove) in
an attempt to allow unshelving into unknown files.  This produced unexpected
results, like the file time stamp changing and a .i file being created.

This change makes it no longer use addremove.  It ignores unknown files
completely.  If an unshelve would overwrite an unknown file, the unknown file is
moved to *.orig

The shelve continue/abort format is changed, but it just removes stuff from the
end of the file, so it can still read the old format.

try:
    import coverage
    getattr(coverage, 'process_startup', lambda: None)()
except ImportError:
    pass