build: pin diff.renames for deterministic patch export (#50121)

git format-patch honors diff.renames, which defaults to 'true' (rename
detection only). If a user has diff.renames=copies configured at the
system or global level, exported patches may encode new files as copies
of similar existing files, causing spurious diffs against patches
exported on other machines. Pin diff.renames=true to match git's
default.

script/lib/git.py

@@ -128,6 +128,11 @@ def format_patch(repo, since):
         os.path.dirname(os.path.realpath(__file__)),
         'electron.gitattributes',
     ),
+    # Pin rename/copy detection to git's default so that patch output is
+    # deterministic regardless of local or system-level diff.renames config
+    # (e.g. 'copies', which would encode similar new files as copies).
+    '-c',
+    'diff.renames=true',
     # Ensure it is not possible to match anything
     # Disabled for now as we have consistent chunk headers
     # '-c',