share: show warning if share is outdated while source supports share-safe
Previous patches in the series and some which are already committed implements
share safe functionality where config and requirements will be shared too.
Rolling this feature has a problem that existing shares may never upgrade as
they will never learn about the new config. To help the transition, we show a
warning message if the share source supports share-safe mechanism. This provides
the source repo ability to upgrade and pass on the message to shares that you
should reshare and upgrade too.
Differential Revision: https://phab.mercurial-scm.org/D9369
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