CI: Use git diff to check 'go mod tidy', 'go fmt'

Rather than capturing the output of these commands directly, we'll
inspect their effect on the working copy. This should provide more
useful information anyway.

Also, I decided to keep the "Install dependencies" step, even though 'go
mod tidy' will install everything. I think it's worth seeing what's used
by the main program vs. the tests and build tools.

.github/workflows/ci.yml

@@ -19,11 +19,13 @@ jobs:
 
       - name: Check that go.mod is tidy
         run: |
-          result="$(go mod tidy -x -v 2>&1)" && echo "$result" && [[ -z "$result" ]]
+          go mod tidy
+          result="$(git diff)" && echo "$result" && [[ -z "$result" ]]
 
       - name: Run static checks
         run: |
-          result="$(go fmt ./...)" && echo "$result" && [[ -z "$result" ]]
+          go fmt ./...
+          result="$(git diff)" && echo "$result" && [[ -z "$result" ]]
           go vet ./...
           go run honnef.co/go/tools/cmd/staticcheck ./...