tool/update_tool: new tool to explore db updates

Db updates are tricky to review. This tools brings some utility
functions to try to simplify the process. It gives you access to 3
commands: Ls, Diff and Report

List the changed DB entry and know if the manifest or the image info was
updated easily. The tool also ignores by default the volatiles UUID so
you are not polluted by that during the review process.

Diff the changed files, either all of them one after the other or a
specific set given by the user. It will open a difftool (vimdiff by
default) to diff either the manifest or the image-info (or both if they
both changed).

Report the change as an ascii table with color codes (by default) or as
a github TODO list to be integrated within the github PR.