sharesafe: make warning about outdated share configurable
If the source repository upgrades to use sharesafe mode, we show a warning in
shares. This patch makes that warning configurable and some might not want their
users see this warning.
Differential Revision: https://phab.mercurial-scm.org/D9691
edition = "2018"
max_width = 79
wrap_comments = true
error_on_line_overflow = true