bookmark: read pending data when appropriate If we are called by a hook and pending data exists, read it.

test-bundle2: check visible data in pre/post-transaction hooks We are about to make bookmarks and phases available for hooks. Therefore we need a witness for this new availability. We introduce the new hooks in a distinct changeset to reduce the noise in the ones with actual changes.

transaction: write pending generated files Such file are generated with a .pending prefix. It is up to the reader to implement the necessary logic for reading pending files. We add a test to ensure pending files are properly cleaned-up in both success and error cases.

transaction: have _generatefile return a boolean The function returns True if any files were generated. This will be used to know if any pending files have been written.