Abbreviation of presets in code signing

Perhaps it would be better if presets for code signing (`dropbox`, `git`, etc) were included as is, i.e. as a list of ignored files/directories/patterns.

Right now most information required to verify is contained in the SIGNED.md file itself, but not the ignored files (only a preset string representing them).

Two possible complications:
1. Dropbox ignored files may change, as Dropbox et. al. evolves. The preset may become outdated, but it would not be obvious from the signed file.
2. Writing a script (or another client) to verify files require the clients to agree on which files are ignored based on a given present string.

Start with SIGNED.md and the current code-signing preset representation, then trace how ignored files and directories are selected and recorded. Compare the Dropbox and Git presets and determine what signed output an independent verifier would need; done means the ignored paths are explicit and preset changes are visible.

Written by the indexing model from the issue text.