Fix fastforward git user configuration

The recent git auto fast forward jobs fails because of the misconfigured
git user:

https://storage.googleapis.com/kubernetes-jenkins/logs/ci-fast-forward/1516649755411746816/build-log.txt

```
Step #3: level=info msg="Merging main branch changes into release branch"
Step #3: level=fatal msg="merge main ref: run git merge: command /usr/bin/git merge -X ours origin/master did not succeed: Committer identity unknown\n\n*** Please tell me who you are.\n\nRun\n\n  git config --global user.email \"you@example.com\"\n  git config --global user.name \"Your Name\"\n\nto set your account's default identity.\nOmit --global to set the identity only in this repository.\n\nfatal: unable to auto-detect email address (got 'root@c96ac26cb82d.(none)')\n"
Finished Step #3
ERROR
```

We now fix this by doing the same configuration like we do in krel
stage and release.

Signed-off-by: Sascha Grunert <sgrunert@redhat.com>

pkg/fastforward/fastforward.go

@@ -203,6 +203,11 @@ func (f *FastForward) Run() (err error) {
 	}
 	logrus.Infof("Latest release branch revision is %s", releaseRev)
 
+	logrus.Info("Configuring git user and email")
+	if err := f.ConfigureGlobalDefaultUserAndEmail(); err != nil {
+		return errors.Wrap(err, "configure git user and email")
+	}
+
 	logrus.Info("Merging main branch changes into release branch")
 	if err := f.RepoMerge(repo, f.options.MainRef); err != nil {
 		return errors.Wrap(err, "merge main ref")

pkg/fastforward/fastforward_test.go

@@ -275,6 +275,17 @@ func TestRun(t *testing.T) {
 				require.NotNil(t, err)
 			},
 		},
+		{ // failure on ConfigureGlobalDefaultUserAndEmail
+			prepare: func(mock *fastforwardfakes.FakeImpl) *Options {
+				mock.IsReleaseBranchReturns(true)
+				mock.RepoHasRemoteBranchReturns(true, nil)
+				mock.ConfigureGlobalDefaultUserAndEmailReturns(errTest)
+				return &Options{Branch: branch}
+			},
+			assert: func(err error) {
+				require.NotNil(t, err)
+			},
+		},
 		{ // failure on RepoMergeBase
 			prepare: func(mock *fastforwardfakes.FakeImpl) *Options {
 				mock.IsReleaseBranchReturns(true)

pkg/fastforward/impl.go

@@ -57,6 +57,7 @@ type impl interface {
 	RemoveAll(string) error
 	MkdirTemp(string, string) (string, error)
 	Exists(string) bool
+	ConfigureGlobalDefaultUserAndEmail() error
 }
 
 func (*defaultImpl) CloneOrOpenDefaultGitHubRepoSSH(repo string) (*git.Repo, error) {
@@ -158,3 +159,7 @@ func (*defaultImpl) MkdirTemp(dir, pattern string) (string, error) {
 func (*defaultImpl) Exists(path string) bool {
 	return util.Exists(path)
 }
+
+func (*defaultImpl) ConfigureGlobalDefaultUserAndEmail() error {
+	return git.ConfigureGlobalDefaultUserAndEmail()
+}