rebase: correct the return values for CurrentOperationIndex

We were incorectly reporting `C.GIT_REBASE_NO_OPERATION` as an error code when
it is none. We should instead return it as the value. The compiler doesn't seem
to actually look at the sizes so instead we must recreate the value ourselves
with `^uint(0)`.

The error return is kept for API compatibility but should go away eventually.

rebase.go

@@ -5,6 +5,7 @@ package git
 */
 import "C"
 import (
+	"errors"
 	"runtime"
 	"unsafe"
 )
@@ -25,6 +26,12 @@ const (
 	RebaseOperationExec RebaseOperationType = C.GIT_REBASE_OPERATION_EXEC
 )
 
+// Special value indicating that there is no currently active operation
+var RebaseNoOperation uint = ^uint(0)
+
+// Error returned if there is no current rebase operation
+var ErrRebaseNoOperation = errors.New("o current rebase operation")
+
 // RebaseOperation describes a single instruction/operation to be performed during the rebase.
 type RebaseOperation struct {
 	Type RebaseOperationType
@@ -154,18 +161,21 @@ func (rebase *Rebase) OperationAt(index uint) *RebaseOperation {
 	return newRebaseOperationFromC(operation)
 }
 
-// CurrentOperationIndex gets the index of the rebase operation that is currently being applied.
+// CurrentOperationIndex gets the index of the rebase operation that is
-// Returns an error if no rebase operation is currently applied.
+// currently being applied. There is also an error returned for API
+// compatibility.
 func (rebase *Rebase) CurrentOperationIndex() (uint, error) {
 	runtime.LockOSThread()
 	defer runtime.UnlockOSThread()
 
-	operationIndex := int(C.git_rebase_operation_current(rebase.ptr))
+	var err error
-	if operationIndex == C.GIT_REBASE_NO_OPERATION {
+	operationIndex := uint(C.git_rebase_operation_current(rebase.ptr))
-		return 0, MakeGitError(C.GIT_REBASE_NO_OPERATION)
+	runtime.KeepAlive(rebase)
+	if operationIndex == RebaseNoOperation {
+		err = ErrRebaseNoOperation
 	}
 
-	return uint(operationIndex), nil
+	return uint(operationIndex), err
 }
 
 // OperationCount gets the count of rebase operations that are to be applied.

rebase_test.go

@@ -182,7 +182,7 @@ func performRebaseOnto(repo *Repository, branch string) (*Rebase, error) {
 
 	// Check no operation has been started yet
 	rebaseOperationIndex, err := rebase.CurrentOperationIndex()
-	if err == nil {
+	if rebaseOperationIndex != RebaseNoOperation && err != ErrRebaseNoOperation {
 		return nil, errors.New("No operation should have been started yet")
 	}