Enhancement: Add negative patterns for include/exclude

If a pattern is suffixed by an exclamation mark and match a file that
was previously matched by a regular pattern, the match is cancelled.
Notably, this can be used with `--exclude-file` to cancel the
exclusion of some files.

It works similarly to `gitignore`, with the same limitation: once a
directory is excluded, it is not possible to include files inside the
directory.

Example of use (as an exclude pattern for backup):

    $HOME/**/*
    !$HOME/Documents
    !$HOME/code
    !$HOME/.emacs.d
    !$HOME/games
    # [...]
    node_modules
    *~
    *.o
    *.lo
    *.pyc
    # [...]
    $HOME/code/linux/*
    !$HOME/code/linux/.git
    # [...]

https://github.com/restic/restic/issues/233
https://github.com/restic/restic/pull/2311