Improve on pip-compile to only run pip-compile a second time when the requirements.txt has been changed

[jerbob92]

Code improvement description

I have done improvements on pip-compile before: #5905
pip-compile is amazingly slow since that the backtracking resolver has been made the default and I'm trying to do some speed improvements to make it better for us. Dependency updates can run for multiple hours for us (sometimes even 10+ hours). Folks at pip are also trying to make speed improvements.

One thing we could potentially improve on is only running pip-compile the second time if there's actually a change made in the requirements.txt file. If there is no change in the requirements.txt file, it means no upgrade was done, in that case we can just run write_original_manifest_files and move on, without running pip-compile again.

I also wonder if write_original_manifest_files is still necessary here, because when I run pip-compile -P {dependency} myself, it won't add it to the requirements.in file, which I believe is why that line is in there.

Relevant code is here: https://github.com/dependabot/dependabot-core/blob/main/python/lib/dependabot/python/update_checker/pip_compile_version_resolver.rb#L111

What do you think @Nishnha @jurre ?