Give each const group a type

This allows us to restrict which constants the compiler will allow
through, and makes the sorting in the documentation better.

reference.go

@@ -11,9 +11,10 @@ import (
 	"unsafe"
 )
 
+type ReferenceType int
 const (
-	ReferenceSymbolic = C.GIT_REF_SYMBOLIC
-	ReferenceOid      = C.GIT_REF_OID
+	ReferenceSymbolic ReferenceType = C.GIT_REF_SYMBOLIC
+	ReferenceOid                    = C.GIT_REF_OID
 )
 
 type Reference struct {
@@ -103,8 +104,8 @@ func (v *Reference) Name() string {
 	return C.GoString(C.git_reference_name(v.ptr))
 }
 
-func (v *Reference) Type() int {
-	return int(C.git_reference_type(v.ptr))
+func (v *Reference) Type() ReferenceType {
+	return ReferenceType(C.git_reference_type(v.ptr))
 }
 
 func (v *Reference) Free() {

reference_test.go

@@ -144,7 +144,7 @@ func compareStringList(t *testing.T, expected, actual []string) {
 	}
 }
 
-func checkRefType(t *testing.T, ref *Reference, kind int) {
+func checkRefType(t *testing.T, ref *Reference, kind ReferenceType) {
 	if ref.Type() == kind {
 		return
 	}

tree.go

@@ -13,8 +13,9 @@ import (
 	"unsafe"
 )
 
+type Filemode int
 const (
-	FilemodeNew            = C.GIT_FILEMODE_NEW
+	FilemodeNew   Filemode = C.GIT_FILEMODE_NEW
 	FilemodeTree           = C.GIT_FILEMODE_TREE
 	FilemodeBlob           = C.GIT_FILEMODE_BLOB
 	FilemodeBlobExecutable = C.GIT_FILEMODE_BLOB_EXECUTABLE

walk.go

@@ -13,11 +13,12 @@ import (
 
 // RevWalk
 
+type SortType uint
 const (
-	SortNone        = C.GIT_SORT_NONE
-	SortTopological = C.GIT_SORT_TOPOLOGICAL
-	SortTime        = C.GIT_SORT_TIME
-	SortReverse     = C.GIT_SORT_REVERSE
+	SortNone SortType = C.GIT_SORT_NONE
+	SortTopological   = C.GIT_SORT_TOPOLOGICAL
+	SortTime          = C.GIT_SORT_TIME
+	SortReverse       = C.GIT_SORT_REVERSE
 )
 
 type RevWalk struct {
@@ -81,7 +82,7 @@ func (v *RevWalk) Iterate(fun RevWalkIterator) (err error) {
 	return nil
 }
 
-func (v *RevWalk) Sorting(sm uint) {
+func (v *RevWalk) Sorting(sm SortType) {
 	C.git_revwalk_sorting(v.ptr, C.uint(sm))
 }