Rewrite download_changesets.sh

[grischard]

• Use curl instead of wget to fetch the list of changesets.
• Use mktemp instead of a hardcoded filename for the temporary file.
• Allow the output directory to be specified via a command-line argument.
• Add logging with detailed messages, including timestamps and errors.
• Implement retries with exponential backoff for failed downloads.
• Implement a dry-run mode to display the changesets that would be downloaded without actually downloading them.
• Allow users to specify a custom log file location.
• Output a summary report of the downloaded changesets, including total count and size.
• Implement a help message that displays the available options and their descriptions.

This hopefully makes download_changesets more flexible, robust, and user-friendly than the original.