The reason is that our custom Setup.hs unconditionally tries to install
the darcs man page. It should instead only do that if the darcs
executable is built.

This currently makes it unnecessarily hard for me to keep darcsden
up-to-date, because the simplest way to do that is to use a
cabal.project file (for darcsden) that points to an up-to-date clone of
darcs itself. But cabal new-build will only build the library component
of such a source dependency. My work-around is to comment out the line

  postBuild = \ _ _ _ lbi -> buildManpage lbi,

in Setup.hs.

resolved by

patch 592492b6b0ca524cf154ede4fcb0b902eb596ad0
Author: Ben Franksen <ben.franksen@online.de>
Date:   Wed Feb 12 12:58:10 CET 2020
  * fix manpage generation in Setup.hs and clean up imports