Pass only Git-tracked Go files to gofumpt

Author: kyu08

CONTRIBUTING.md

@@ -109,7 +109,7 @@ by setting [`formatting.gofumpt`](https://github.com/golang/tools/blob/master/go
 To run gofumpt from your terminal go:
 
 ```
-go install mvdan.cc/gofumpt@latest && gofumpt -l -w .
+go install mvdan.cc/gofumpt@latest && make format
 ```
 
 ## Programming Font

Makefile

@@ -34,9 +34,11 @@ test: unit-test integration-test-all
 generate:
 	go generate ./...
 
+# If you execute `gofumpt -l -w .`, it will format all Go files in the current directory, including `test/_results/*` files.
+# We pass only Git-tracked Go files to gofumpt because we don't want to format the test results or get errors from it.
 .PHONY: format
 format:
-	gofumpt -l -w .
+	git ls-files '*.go' ':!vendor' | xargs gofumpt -l -w
 
 .PHONY: lint
 lint: