Stop replacing files in LFS storage when downloading them concurrently on Windows

[slonopotamus]

On Windows, there is no way to replace file atomically. Instead, MoveFileExA:

1. Calls CreateFileA(access=Delete, shareMode=Delete)
2. Calls SetRenameInformationFile to rename file
3. Calls CloseFile

The problem is that if parallel process attempts to open destination file for reading between
steps 2 and 3, it will try to do that without shareMode=Delete and will hit a SHARING_VIOLATION error.

In practice, this race condition results in:

Smudge error: Error opening media file.: open .git\lfs\objects<sha>: The process cannot access the file because it is being used by another process.

There are two solutions here:

1. Do not overwrite file if it already exists
2. Retry reading from file if got a SHARING_VIOLATION

This commit implements option 1.

Fixes #2825 (for Windows, other OSes are race-free already). Also see #3813 and #3826.

[slonopotamus]

FTR, here's an example program (Java) that reproduces described SHARING_VIOLATION race:

import java.io.File;
import java.io.FileInputStream;
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.StandardCopyOption;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.TimeUnit;

public class Main {

  public static void main(String[] args) {
    final int processors = Runtime.getRuntime().availableProcessors();
    final ExecutorService executorService = Executors.newFixedThreadPool(processors);

    final File tmpDir = new File("tmp");

    if (!tmpDir.isDirectory() && !tmpDir.mkdirs())
      System.exit(1);

    final Path dest = new File(tmpDir, "dest").toPath();

    for (int i = 0; i < processors; ++i)
      executorService.submit(() -> {
        while (true) {
          final Path file = File.createTempFile("bla", "", tmpDir).toPath();
          try {
            Files.move(file, dest, StandardCopyOption.ATOMIC_MOVE, StandardCopyOption.REPLACE_EXISTING);
          } catch (Exception e) {
            Files.delete(file);
          }

          TimeUnit.MILLISECONDS.sleep(10);

          try (FileInputStream in = new FileInputStream(dest.toFile())) { // Fails here
            // noop, we just want to open file
          } catch (Exception e) {
            e.printStackTrace();
            System.exit(3);
          }
        }
      });
  }
}

[slonopotamus]

Rebased on top of #3878 to pass CI.

[bk2204]

Yeah, this seems fine. Thanks for the patch.