util: add `nb_bytes` argument to `copyfile` to partially copy a file
When set, this allow to copy only the first `nb_bytes` of a file. This will be
useful for censor/strip operation with revlogv2.
Differential Revision: https://phab.mercurial-scm.org/D10798
edition = "2018"
max_width = 79
wrap_comments = true
error_on_line_overflow = true