Oid: avoid unnecessary copy

Going through GoByte() means we allocate and copy an extra slice that we
just use in order to call copy(). Use memcpy() instead to copy directly
from the git_oid to the Oid.

git.go

@@ -3,7 +3,7 @@ package git
 /*
 #cgo pkg-config: libgit2
 #include <git2.h>
-#include <git2/errors.h>
+#include <string.h>
 */
 import "C"
 import (
@@ -38,7 +38,8 @@ func newOidFromC(coid *C.git_oid) *Oid {
 	}
 
 	oid := new(Oid)
-	copy(oid[0:20], C.GoBytes(unsafe.Pointer(coid), 20))
+
+	C.memcpy(unsafe.Pointer(oid), unsafe.Pointer(coid), 20)
 	return oid
 }