tree: don't use system stat constants

On most Unix systems, the stat constants (S_IFMT and friends) that Git
uses are the same: that is, the system values are the same ones that Git
uses.  However, on zOS, this isn't the case, and the system constants
are different.

Let's add a set of our constants that are used by Git to determine the
type of an object that will be right on all platforms instead of using
the system constants.  Note that the tests already cover this case;
they'll just now pass on additional systems.

Author: bk2204

tree.go

@@ -9,11 +9,21 @@ import (
 	"sort"
 	"strconv"
 	"strings"
-	"syscall"
 
 	"github.com/git-lfs/gitobj/v2/pack"
 )
 
+// We define these here instead of using the system ones because not all
+// operating systems use the traditional values.  For example, zOS uses
+// different values.
+const (
+	sIFMT      = int32(0170000)
+	sIFREG     = int32(0100000)
+	sIFDIR     = int32(0040000)
+	sIFLNK     = int32(0120000)
+	sIFGITLINK = int32(0160000)
+)
+
 // Tree encapsulates a Git tree object.
 type Tree struct {
 	// Entries is the list of entries held by this tree.
@@ -209,24 +219,18 @@ func (e *TreeEntry) Equal(other *TreeEntry) bool {
 // Type is the type of entry (either blob: BlobObjectType, or a sub-tree:
 // TreeObjectType).
 func (e *TreeEntry) Type() ObjectType {
-	switch e.Filemode & syscall.S_IFMT {
-	case syscall.S_IFREG:
+	switch e.Filemode & sIFMT {
+	case sIFREG:
 		return BlobObjectType
-	case syscall.S_IFDIR:
+	case sIFDIR:
 		return TreeObjectType
-	case syscall.S_IFLNK:
+	case sIFLNK:
 		return BlobObjectType
+	case sIFGITLINK:
+		return CommitObjectType
 	default:
-		if e.Filemode == 0xe000 {
-			// Mode 0xe000, or a gitlink, has no formal filesystem
-			// (`syscall.S_IF<t>`) equivalent.
-			//
-			// Safeguard that catch here, or otherwise panic.
-			return CommitObjectType
-		} else {
-			panic(fmt.Sprintf("gitobj: unknown object type: %o",
-				e.Filemode))
-		}
+		panic(fmt.Sprintf("gitobj: unknown object type: %o",
+			e.Filemode))
 	}
 }