working_copy: record expected written size, not actual size, on checkout

When we have just written a file to disk on checkout, let's record the
size we expected instead of what we got from `fstat()`. This should
address a race where the file was modified between the time we wrote
it and the time we requested its stat. I just happened to notice this
while going through the code; it seems very unlikely to be noticed in
practice.

lib/src/working_copy.rs

@@ -608,10 +608,11 @@ impl TreeState {
                 err,
             })?;
         let mut contents = self.store.read_file(path, id)?;
-        std::io::copy(&mut contents, &mut file).map_err(|err| CheckoutError::IoError {
-            message: format!("Failed to write file {}", disk_path.display()),
-            err,
-        })?;
+        let size =
+            std::io::copy(&mut contents, &mut file).map_err(|err| CheckoutError::IoError {
+                message: format!("Failed to write file {}", disk_path.display()),
+                err,
+            })?;
         self.set_executable(disk_path, executable)?;
         // Read the file state from the file descriptor. That way, know that the file
         // exists and is of the expected type, and the stat information is most likely
@@ -625,6 +626,7 @@ impl TreeState {
         // for Windows, since the executable bit is not reflected in the file system
         // there.
         file_state.mark_executable(executable);
+        file_state.size = size;
         Ok(file_state)
     }