Support for pre-commit hooks

[ben-edna]

To guard against unwanted changes, it should be possible to create a pre-commit hook that checks if a file/folder is under control of dfetch.

[spoorcc]

Introduce a new filter command

• dfetch filter can wrap a tool.
  • Its only role is to filter the file paths passed to the tool.
  • It removes any paths:
    • Outside the top-level manifest directory
    • Or that do not match the filter (--inside / --not-inside)
  • After filtering, it executes the tool with the remaining paths.
• The tool itself is unaware of dfetch’s filtering logic
• Can also be used as part of pipe, e.g. find . | dfetch filter
• can be used in pre-commit hooks: dfetch filter --outside black, which will run black on files outside any dfetched projects

[spoorcc]

Instead of --outside use --dfetched and --not-dfetched flags for dfetch filter, with --dfetched as the default.

This reads more natural. Filter keeps what is specified:

• dfetch filter --dfetched means, keep only files that were dfetched
• dfetch filter --not-dfetched means, keep only files that were not dfetched.