Reject merge conflicts

They're usually found by other checks, but docs would remain
susceptible.

(cherry picked from commit 29a1a21ce4)

maintainers/flake-module.nix

@@ -10,6 +10,27 @@
     # https://flake.parts/options/pre-commit-hooks-nix.html#options
     pre-commit.settings = {
       hooks = {
+        # Conflicts are usually found by other checks, but not those in docs,
+        # and potentially other places.
+        check-merge-conflicts.enable = true;
+        # built-in check-merge-conflicts seems ineffective against those produced by mergify backports
+        check-merge-conflicts-2 = {
+          enable = true;
+          entry = "${pkgs.writeScript "check-merge-conflicts" ''
+            #!${pkgs.runtimeShell}
+            conflicts=false
+            for file in "$@"; do
+              if grep --with-filename --line-number -E '^>>>>>>> ' -- "$file"; then
+                conflicts=true
+              fi
+            done
+            if $conflicts; then
+              echo "ERROR: found merge/patch conflicts in files"
+              exit 1
+            fi
+            touch $out
+          ''}";
+        };
         clang-format = {
           enable = true;
           excludes = [