Log Message:
net/unison-snapshot: Update to 2.51.3.72.90

Upstream is about to release 2.51.4rc3 with version number 2.51.3.73,
and this packages a draft of that.

Packaging changes: Don't install the now-missing BUGS.txt.

Upstream NEWS for 2.51.4 relative to 2.51.3:

\item OCaml 4.12 support
\item fsmonitor improvements and Solaris support
\item Color support in text UI, with a new preference, disabled by
  NO\_COLOR.
\item Interactive profile selection in text UI, enabled by a new
  preference.
\item Working files are stored in the unison directory (typically
  ~/.unison) rather than \$HOME.
\item Build cleanups, CI improvements, housekeeping
\item Many bugfixes and minor improvements