Change delta encoding of list deletions

Change delta encoding of list deletions

When encoding list_del commands in delta files, '_' is used as prefix but this
breaks any command operating on files starting with an underscore since the
command prefix becomes indistinguishable from the filename.

The regression was introduced with ec8cbbbe0f20e67cbd3bd471b9ac55ceacaef7b8.

Avoid this problem by using '~' as command prefix for list deletions.