Fix hooksPath when configured from a subdirectory

Resolves #358.

modules/pre-commit.nix

@@ -465,7 +465,17 @@ in
                 else
                   pre-commit install
                 fi
-                ${git}/bin/git config --local core.hooksPath "$(${git}/bin/git rev-parse --git-common-dir)/hooks"
+
+                # Fetch the relative path to the root of the git repository
+                cdup=''$(${git}/bin/git rev-parse --show-cdup)
+                # Fetch the relative path to the git common directory. Normally set to .git.
+                common_dir=''$(${git}/bin/git rev-parse --path-format=relative --git-common-dir)
+
+                # When run in a subdirectory, the common directory will be a relative path up from that subdirectory.
+                # What we want instead is a path to the common directory, relative to the root.
+                common_dir=''${common_dir#''$cdup}
+
+                ${git}/bin/git config --local core.hooksPath "''$common_dir/hooks"
               fi
             fi
           fi