prek: fix an issue with prek 0.3.9+ failing to resolve the hooks dir

Prek doesn't like an empty string for `hooksPath`.

Fixes #707.

Author: sandydoo

modules/pre-commit.nix

@@ -510,7 +510,8 @@ in
               for hook in $hooks; do
                 ${lib.getExe cfg.package} uninstall -t $hook
               done
-              ${lib.getExe cfg.gitPackage} config --local core.hooksPath ""
+              # Clear any user-configured core.hooksPath so the hook tool installs into the real hooks dir.
+              ${lib.getExe cfg.gitPackage} config --local --unset-all core.hooksPath || true
               # Add hooks for configured stages (only) ...
               if [ ! -z "${concatStringsSep " " install_stages}" ]; then
                 for stage in ${concatStringsSep " " install_stages}; do

nix/tests/installation.nix

@@ -1,12 +1,20 @@
 # Test that checks whether the correct hooks are created in the hooks folder.
-{ git, perl, coreutils, runCommand, run, lib, mktemp }:
+{ git, perl, coreutils, runCommand, run, lib, mktemp, prek }:
 let
   tests = {
     basic-test = {
       expectedHooks = [ "pre-commit" ];
       conf.hooks.shellcheck.enable = true;
     };
 
+    prek-test = {
+      expectedHooks = [ "pre-commit" ];
+      conf = {
+        package = prek;
+        hooks.shellcheck.enable = true;
+      };
+    };
+
     multiple-hooks-test = {
       expectedHooks = [ "commit-msg" "pre-commit" ];
       conf.hooks = {