net/unison-snapshot: Update to 2.51.2.20200915

This is 2.51.3_rc1.  It is believed to interoperate with 2.51.2 (no
wire protocol break), but this is not 100% certain.

Upstream changes relative to the last snapshot:

  (not provided, probably many, mostly bugfixes)
  A change since 2.51.2 caused a protocol break; this has been reverted.

Upstream NEWS relative to the last release:

  Changes since 2.51.2:
    * Some nontrivial changes to profile parsing (G.raud Meyer)
          + ’=’ has been considered whitespace until now: several
            following chars are considered as only one; trailing chars are
            discarded; any non emty sequence of char is splitting. This is
            non standard and leads to confusion, for example -ignore==
            ’Name .*=*’ is valid when -ignore=’Name .*=*’ is not, and
            worse -ignore=’Name *=’ is the same as -ignore=’Name *’. The
            parser now takes just a single ’=’ as delimiter after the
            option name. Other = characters are considered as part of the
            value being assigned to the option.
    * Numerous improvements to the text user-interface (G.raud Meyer)
          + New key-commands that restrict the display to a set of
            "matching" items: ones that are offering to propagate changes
            in a particular direction, conflicts, files to be merged,
            etc., plus several more useful key-commands. Type "?" to
            Unison to see all available commands.

(gdt)