store: cache the `files()` return for store entries

store: cache the `files()` return for store entries

This make it more efficient to directly use the entries list to retrieve data
in various location. It also make the entry record the file size it previously
promissed to user code, especially the stream clone code.

--- a/mercurial/store.py	Sat May 27 04:22:18 2023 +0200
+++ b/mercurial/store.py	Sun May 28 03:46:48 2023 +0200
@@ -475,6 +475,7 @@
     _entry_path = attr.ib()
     _is_volatile = attr.ib(default=False)
     _file_size = attr.ib(default=None)
+    _files = attr.ib(default=None)
 
     def __init__(
         self,
@@ -486,15 +487,18 @@
         self._entry_path = entry_path
         self._is_volatile = is_volatile
         self._file_size = file_size
+        self._files = None
 
     def files(self):
-        return [
-            StoreFile(
-                unencoded_path=self._entry_path,
-                file_size=self._file_size,
-                is_volatile=self._is_volatile,
-            )
-        ]
+        if self._files is None:
+            self._files = [
+                StoreFile(
+                    unencoded_path=self._entry_path,
+                    file_size=self._file_size,
+                    is_volatile=self._is_volatile,
+                )
+            ]
+        return self._files
 
 
 @attr.s(slots=True, init=False)
@@ -507,6 +511,7 @@
     target_id = attr.ib(default=None)
     _path_prefix = attr.ib(default=None)
     _details = attr.ib(default=None)
+    _files = attr.ib(default=None)
 
     def __init__(
         self,
@@ -521,6 +526,7 @@
         self._path_prefix = path_prefix
         assert b'.i' in details, (path_prefix, details)
         self._details = details
+        self._files = None
 
     @property
     def is_changelog(self):
@@ -539,12 +545,13 @@
         return self._path_prefix + b'.i'
 
     def files(self):
-        files = []
-        for ext in sorted(self._details, key=_ext_key):
-            path = self._path_prefix + ext
-            data = self._details[ext]
-            files.append(StoreFile(unencoded_path=path, **data))
-        return files
+        if self._files is None:
+            self._files = []
+            for ext in sorted(self._details, key=_ext_key):
+                path = self._path_prefix + ext
+                data = self._details[ext]
+                self._files.append(StoreFile(unencoded_path=path, **data))
+        return self._files
 
     def get_revlog_instance(self, repo):
         """Obtain a revlog instance from this store entry