more nuanced error typing

git/git.go

@@ -29,10 +29,12 @@ func CurrentBranch() (string, error) {
 		return firstLine(output), nil
 	}
 
-	ce := err.(*run.CmdError)
-	if ce.Stderr.Len() == 0 {
-		// Detached head
-		return "", errors.New("git: not on any branch")
+	var cmdErr *run.CmdError
+	if errors.As(err, &cmdErr) {
+		if cmdErr.Stderr.Len() == 0 {
+			// Detached head
+			return "", errors.New("git: not on any branch")
+		}
 	}
 
 	// Unknown error

test/helpers.go

@@ -14,7 +14,7 @@ import (
 // OutputStub implements a simple utils.Runnable
 type OutputStub struct {
 	Out   []byte
-	Error *run.CmdError
+	Error error
 }
 
 func (s OutputStub) Output() ([]byte, error) {
@@ -49,6 +49,7 @@ func (cs *CmdStubber) Stub(desiredOutput string) {
 }
 
 func (cs *CmdStubber) StubError(errText string) {
+	// TODO support error types beyond CmdError
 	stderrBuff := bytes.NewBufferString(errText)
 	args := []string{"stub"} // TODO make more real?
 	err := errors.New(errText)