subtree: use contains_shallow_copy() in ctx.files()

Summary:
For subtree commits, we only need to compute the file list if it contains shallow copy. So this diffs
check that to determine if the manifest diff is needed. Later, we will make `self._changeset.files`
an empty list if the commit contains a shallow copy, and in long-term, we will remove the file list
from commit text.

Reviewed By: muirdm

Differential Revision: D65462388

fbshipit-source-id: c85e8fa8a1d45807dd100d39990e8ea0b828eceb

eden/scm/sapling/context.py

@@ -55,7 +55,7 @@
     wdirrev,
 )
 from .pycompat import encodeutf8, isint, range
-from .utils.subtreeutil import SUBTREE_BRANCH_KEY
+from .utils import subtreeutil
 
 filectx_or_bytes = Union["basefilectx", bytes]
 
@@ -591,10 +591,9 @@ def files(self):
             # The following cases does not provide "files" in commit message,
             # run diff to get it:
             # - git repo
-            # - O(1) subtree copy
-            if (
-                git.isgitformat(self._repo)
-                or SUBTREE_BRANCH_KEY in self._changeset.extra
+            # - subtree shallow copy
+            if git.isgitformat(self._repo) or subtreeutil.contains_shallow_copy(
+                self._repo, self.node()
             ):
                 files = sorted(self.manifest().diff(self.p1().manifest()).keys())
         return files

eden/scm/sapling/utils/subtreeutil.py

@@ -396,6 +396,14 @@ def is_source_commit_allowed(ui, source_ctx) -> bool:
     return False
 
 
+def contains_shallow_copy(repo, node):
+    branches = get_subtree_branches(repo, node)
+    for b in branches:
+        if b.branch_type == BranchType.SHALLOW_COPY:
+            return True
+    return False
+
+
 def check_commit_splitability(repo, node):
     """Check if the given commit can be split into multiple commits.