wireprotov2: advertise recognized path filter prefixes
While the wire protocol doesn't yet support it, we'll eventually
have commands that accept narrow patterns to specify the set of
files relevant to a command.
For security and performance reasons, only specific filter types
are allowed.
This commit teaches the server to advertise the set of allowed
filter types. By doing so, clients can e.g. validate user-specified
patterns against the server's abilities without having to send
a command to retrieve data.
Having the data in the capabilities data structure will also serve
as a check against unwanted BC.
Differential Revision: https://phab.mercurial-scm.org/D4616
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