programs.git: test git integration options assertion

tests/modules/programs/git/default.nix

@@ -10,4 +10,5 @@
   git-with-lfs = ./git-with-lfs.nix;
   git-with-maintenance = ./git-with-maintenance.nix;
   git-settings-deprecations = ./git-settings-deprecations.nix;
+  git-integration-assertion = ./git-integration-assertion.nix;
 }

tests/modules/programs/git/git-integration-assertion.nix

@@ -0,0 +1,23 @@
+let
+  enable = {
+    enable = true;
+    enableGitIntegration = true;
+  };
+in
+{
+  programs = {
+    delta = enable;
+    # FIXME(leana8959): these two aren't caught by the tests.
+    # diff-highlight = enable;
+    # diff-so-fancy = enable;
+    patdiff = enable;
+  };
+
+  test.asserts.assertions.expected = [
+    ''
+      Only one of the following options can be enabled at a time.
+        - `programs.delta.enableGitIntegration'
+        - `programs.patdiff.enableGitIntegration'
+    ''
+  ];
+}