fish: avoid shadowing builtin completions

The fish shell comes with builtin completions. For example, git
completion supports context-aware completion of things like commit
hashes, branch names, sub-commands, etc.

Until fish 4.2, builtin completions were explicitly loaded from
`share/fish/completions`, however that is now deprecated and disabled.
In effect, this means generating manpage-based completion will shadow
and disable builtin completion.

Avoid that, by only generating completion when fish does not have
builtin support for the command.

modules/programs/fish.nix

@@ -628,9 +628,30 @@ in
                 mkdir -p $out
                 for src in $srcs; do
                   if [ -d $src/share/man ]; then
-                    find -L $src/share/man -type f \
-                      -exec python ${cfg.package}/share/fish/tools/create_manpage_completions.py --directory $out {} + \
-                      > /dev/null
+                    while IFS= read -r manpage; do
+                      # Approximate the corresponding command for this manpage
+                      bin="$(basename "$manpage")"
+                      bin="''${bin%%.*}"
+                      bin="$src/bin/$bin"
+
+                      # Check for builtin completion
+                      if
+                        [ -e "$bin" ] &&
+                        fish \
+                          --no-config \
+                          --command 'complete --do-complete $argv[1]' \
+                          -- "$bin" \
+                          >/dev/null 2>&1
+                      then
+                        echo "Found builtin completion for $bin (skipping)"
+                        continue
+                      fi
+
+                      # Generate completion based on the manpage
+                      python ${cfg.package}/share/fish/tools/create_manpage_completions.py \
+                        --directory "$out" "$manpage" > /dev/null
+
+                    done < <(find -L "$src/share/man" -type f)
                   fi
                 done
               '';