Leave removed copies out of report and trees (closes #173)

A deleted duplicate stayed in both reports until a scan covered it, and
a missing scan operand is a fatal error. report now runs lstat on each
path of a possible duplicate group and drops paths that are gone; trees
does the same for the directory at the top of each copy, never the
files inside. A group left with one copy is not reported. Only "does not
exist" and "not a directory" count as gone, so a permission error keeps
the copy for an unprivileged user. README says a stale report is fixed
by scanning again, and its Docker example mounts the scanned tree for
report too.

Model: opus-5-5

README.md

@@ -99,13 +99,15 @@ Each `-v` flag below makes a bind mount, which shows a host directory at a path
 inside the container. Bind-mount `/var/lib/sfdupes` to keep the database between
 runs. Mount the tree to scan at the same path inside the container as on the
 host. Read-only access is enough. The database then records the host's paths,
-and so do the reports.
+and so do the reports. `report` and `trees` need that mount too. Without it,
+every recorded path counts as gone, and each report holds only its header line.
 
 ```sh
 make docker
 docker run --rm -v /srv:/srv:ro -v /var/lib/sfdupes:/var/lib/sfdupes \
     sfdupes scan /srv
-docker run --rm -v /var/lib/sfdupes:/var/lib/sfdupes sfdupes report > dupes.tsv
+docker run --rm -v /srv:/srv:ro -v /var/lib/sfdupes:/var/lib/sfdupes \
+    sfdupes report > dupes.tsv
 ```
 
 ### Daily scan from `cron`
@@ -738,7 +740,6 @@ when they hold the same relative paths, and each path holds a file with the same
 signature in both. It rebuilds the directory tree from the paths in the records,
 split on `/`. Like `report`, it never opens a scanned file. It checks only that
 the directory at the top of each copy still exists, as `report` checks a path.
-It looks at no file or directory inside a copy.
 
 Definitions: