`detect_incremental(root)` always called `detect(root)` without forwarding
the `follow_symlinks` kwarg. As a result, corpora that include symlinked
sub-trees pointing to directories outside the scan root (e.g. a
`state_of_truth/` symlink pointing at `~/.hermes/state_of_truth/`) were
visible to a full `detect()` run with `follow_symlinks=True` but invisible
to any subsequent `--update` run. The incremental scan would then either
report no changes (silently dropping legitimate new files) or repeatedly
re-extract a phantom subset, depending on what was reachable without
crossing symlinks.
Add a keyword-only `follow_symlinks` parameter to `detect_incremental()`
and forward it. Default stays `False` for backwards compatibility — only
callers that already opt in to symlink following on `detect()` pick up
the new behaviour for incremental runs too.
Test: a corpus with a symlinked directory is invisible with
`follow_symlinks=False`, fully indexed with `follow_symlinks=True`, and
correctly reports zero new files on a second incremental scan after the
manifest is saved.
Safi