Makefile: Use 'git diff' to show gofmt changes

This makes fixing errors easier.  Before this commit, errors looked
like [1]:

  $ make gofmt
  !!! 'gofmt -s' needs to be run on the following files:
  ./lib/config.go
  make: *** [gofmt] Error 1

But that's not very helpful when your local gofmt thinks the file is
fine.  With this commit, errors will look like:

  $ make gofmt
  find . -name '*.go' ! -path './vendor/*' -exec gofmt -s -w {} \+
  git diff --exit-code
  diff --git a/lib/config.go b/lib/config.go
  index 1acca8c7..6a63b2b0 100644
  --- a/lib/config.go
  +++ b/lib/config.go
  @@ -2,7 +2,7 @@ package lib

   import (
          "bytes"
  -"io/ioutil"
  +       "io/ioutil"

          "github.com/BurntSushi/toml"
          "github.com/kubernetes-incubator/cri-o/oci"
  make: *** [Makefile:68: gofmt] Error 1

(or whatever, I just stuffed in a formatting error for demonstration
purposes).

Also remove the helper script in favor of direct Makefile calls,
because with Git handling difference reporting and exit status, this
becomes a simpler check.  find's -exec, !, and -path arguments are
specified in POSIX [2].

[1]: https://travis-ci.org/kubernetes-incubator/cri-o/jobs/331949394#L1075
[2]: http://pubs.opengroup.org/onlinepubs/9699919799/utilities/find.html

Signed-off-by: W. Trevor King <wking@tremily.us>

Makefile

@@ -64,7 +64,8 @@ lint: .gopathok
 	@./.tool/lint
 
 gofmt:
-	@./hack/verify-gofmt.sh
+	find . -name '*.go' ! -path './vendor/*' -exec gofmt -s -w {} \+
+	git diff --exit-code
 
 conmon:
 	$(MAKE) -C $@

hack/verify-gofmt.sh

@@ -1,21 +0,0 @@
-#!/bin/bash
-
-set -o errexit
-set -o nounset
-set -o pipefail
-
-find_files() {
-  find . -not \( \
-      \( \
-        -wholename '*/vendor/*' \
-      \) -prune \
-    \) -name '*.go'
-}
-
-GOFMT="gofmt -s"
-bad_files=$(find_files | xargs $GOFMT -l)
-if [[ -n "${bad_files}" ]]; then
-  echo "!!! '$GOFMT' needs to be run on the following files: "
-  echo "${bad_files}"
-  exit 1
-fi