fix(plugins): support newer git (#3571)

Newer gits don't like checking out an empty string.

```
empty string is not a valid pathspec. please use . instead if you meant to match all paths
```

pkg/plugin/installer/vcs_installer.go

@@ -78,9 +78,10 @@ func (i *VCSInstaller) Install() error {
 	if err != nil {
 		return err
 	}
-
-	if err := i.setVersion(i.Repo, ref); err != nil {
-		return err
+	if ref != "" {
+		if err := i.setVersion(i.Repo, ref); err != nil {
+			return err
+		}
 	}
 
 	if !isPlugin(i.Repo.LocalPath()) {

pkg/plugin/installer/vcs_installer_test.go

@@ -69,20 +69,20 @@ func TestVCSInstaller(t *testing.T) {
 
 	i, err := NewForSource(source, "~0.1.0", home)
 	if err != nil {
-		t.Errorf("unexpected error: %s", err)
+		t.Fatalf("unexpected error: %s", err)
 	}
 
 	// ensure a VCSInstaller was returned
 	vcsInstaller, ok := i.(*VCSInstaller)
 	if !ok {
-		t.Error("expected a VCSInstaller")
+		t.Fatal("expected a VCSInstaller")
 	}
 
 	// set the testRepo in the VCSInstaller
 	vcsInstaller.Repo = repo
 
 	if err := Install(i); err != nil {
-		t.Error(err)
+		t.Fatal(err)
 	}
 	if repo.current != "0.1.1" {
 		t.Errorf("expected version '0.1.1', got %q", repo.current)
@@ -123,13 +123,13 @@ func TestVCSInstallerNonExistentVersion(t *testing.T) {
 
 	i, err := NewForSource(source, version, home)
 	if err != nil {
-		t.Errorf("unexpected error: %s", err)
+		t.Fatalf("unexpected error: %s", err)
 	}
 
 	// ensure a VCSInstaller was returned
 	_, ok := i.(*VCSInstaller)
 	if !ok {
-		t.Error("expected a VCSInstaller")
+		t.Fatal("expected a VCSInstaller")
 	}
 
 	if err := Install(i); err == nil {
@@ -155,40 +155,40 @@ func TestVCSInstallerUpdate(t *testing.T) {
 
 	i, err := NewForSource(source, "", home)
 	if err != nil {
-		t.Errorf("unexpected error: %s", err)
+		t.Fatalf("unexpected error: %s", err)
 	}
 
 	// ensure a VCSInstaller was returned
 	_, ok := i.(*VCSInstaller)
 	if !ok {
-		t.Error("expected a VCSInstaller")
+		t.Fatal("expected a VCSInstaller")
 	}
 
 	if err := Update(i); err == nil {
-		t.Error("expected error for plugin does not exist, got none")
+		t.Fatal("expected error for plugin does not exist, got none")
 	} else if err.Error() != "plugin does not exist" {
-		t.Errorf("expected error for plugin does not exist, got (%v)", err)
+		t.Fatalf("expected error for plugin does not exist, got (%v)", err)
 	}
 
 	// Install plugin before update
 	if err := Install(i); err != nil {
-		t.Error(err)
+		t.Fatal(err)
 	}
 
 	// Test FindSource method for positive result
 	pluginInfo, err := FindSource(i.Path(), home)
 	if err != nil {
-		t.Error(err)
+		t.Fatal(err)
 	}
 
 	repoRemote := pluginInfo.(*VCSInstaller).Repo.Remote()
 	if repoRemote != source {
-		t.Errorf("invalid source found, expected %q got %q", source, repoRemote)
+		t.Fatalf("invalid source found, expected %q got %q", source, repoRemote)
 	}
 
 	// Update plugin
 	if err := Update(i); err != nil {
-		t.Error(err)
+		t.Fatal(err)
 	}
 
 	// Test update failure

pkg/storage/driver/records_test.go

@@ -78,7 +78,7 @@ func TestRecordsRemove(t *testing.T) {
 	for _, tt := range tests {
 		if r := rs.Remove(tt.key); r == nil {
 			if !tt.ok {
-				t.Fatalf("Failed to %q (key = %s). Expected nil, got %s",
+				t.Fatalf("Failed to %q (key = %s). Expected nil, got %v",
 					tt.desc,
 					tt.key,
 					r,