Log Message:
net/unison2.51: Update to 2.52.0

## Changes in 2.52.0

Released 2022-03-12

   * Feature negotiation, compatible wtih 2.51.
   * New archive format (independent of ocaml version, based on umarshal)
     Upgrade is automatic.
   * New wire protocol (independent of ocaml version, based on umarshal)
     New protocol is used if both sides are >= 2.52.0.
   * Compatibility with 2.48
   * Support for unix-domain sockets
   * Many bugfixes and minor improvements
   * ocaml compatibility is now >= 4.01
   * NEWS is now in NEWS.md and not in the manual