branchcache: add debug output whenever cache files use truncate The cache files are usually append only but will automatically be truncated and recover in exceptional situations. Add a debug notice when such exceptional situations are encountered.

tests: rework revision branch cache tests The tests that were added with the revision branch cache in 678f53865c68 had suffered from bit rot in the development iterations. They were no longer that "good". Now, the tests are rewritten and reworked to be more readable and maintainable and relevant for the actual implementation. This also utilizes the new 'f' helper tool for keeping an eye the cache files.

tests: add 'f' tool for cross platform file operations in the tests This tool is like the collection of tools found in a unix environment but are cross platform and stable and suitable for our needs in the test suite. The main reason it is "needed" now is for hexdump of revision branch cache to keep an eye on how it changes and make sure the format is stable. It is a very generic tool that can end up being used a lot in tests, so I gave it very generic name.