model: avoid another extra creation of Model object

Do not create model object while sorting entry before creating model
object.

This is another case of "let us create object X to prepare the creation
of object X", slowing things down.

In practice, we will likely skip this code-path after the next
changeset, however this seems useful to get this performance footgun
out the way.

We tested this change on simple information of the Mercurial loader,
with a noop-loader stockage:

    swh loader run mercurial https://foss.heptapod.net/mercurial/mercurial-devel directory=/data/repos/mercurial-devel

= Median time of 3 run =
before  12 minutes 59 seconds
after:  11 minute  56 seconds

On a profile of the same run, the `to_model` call of the from_disk's `Directory` class took the following percentage:
before: 24%
after:  17%

swh/model/git_objects.py

@@ -55,12 +55,16 @@ def content_git_object(content: model.Content) -> bytes:
 def directory_entry_sort_key(entry: model.DirectoryEntry):
     """The sorting key for tree entries"""
     if isinstance(entry, dict):
-        # For backward compatibility
-        entry = model.DirectoryEntry.from_dict(entry)
-    if entry.type == "dir":
-        return entry.name + b"/"
+        type_ = entry["type"]
+        name = entry["name"]
+    else:
+        type_ = entry.type
+        name = entry.name
+
+    if type_ == "dir":
+        return name + b"/"
     else:
-        return entry.name
+        return name
 
 
 @lru_cache()