fix(patching): fall back to copy when hard linking fails in patch-commit (#15395)

In environments where hard linking fails (such as cross-device
boundaries or filesystems without hard link support), fall back to
copying package files when preparing the diff view.

Closes pnpm/pnpm#7295

.changeset/patch-commit-exdev-fallback.md

@@ -0,0 +1,7 @@
+---
+"@pnpm/patching.commands": patch
+"pnpm": patch
+"pacquet": patch
+---
+
+`pnpm patch-commit` now falls back to copying package files when hard linking fails.