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

@@ -22,8 +22,9 @@ records the files it finds in an SQLite database. SQLite keeps a whole database
 in one file. The database is kept between runs. A rescan reads only new and
 changed files. That makes `scan` cheap enough to run every day from `cron`, the
 system's job scheduler. `report`, `trees` and `info` work from the database and
-can run at any time. `dwim` scans `/` and then prints the `trees` report, for a
-first look at a new machine with one command.
+can run at any time. `report` and `trees` leave out a copy that was deleted
+after the scan. `dwim` scans `/` and then prints the `trees` report, for a first
+look at a new machine with one command.
 
 ## Getting Started
 
@@ -114,20 +115,15 @@ docker run --rm -v /srv:/srv:ro -v /var/lib/sfdupes:/var/lib/sfdupes \
 Run `scan` as root, so that it can read every file. `scan` warns about a
 directory it cannot read, and about a new or changed file it cannot read. It
 skips the path and deletes the records at or below it (see "Rules for the
-walk").
-
-An unmounted disk leaves its mount point empty, so `scan` deletes every record
-under it, and the next scan reads every file on the disk again. Test the mount
-point of each disk at or below `/srv` with its own `mountpoint -q`, joined with
-`&&`, and test `/srv` only when it is one. As the file `/etc/cron.d/sfdupes`:
+walk"). As the file `/etc/cron.d/sfdupes`:
 
 ```
-30 3 * * * root if mountpoint -q /srv; then /usr/local/bin/sfdupes scan /srv 2>>/var/log/sfdupes.log || tail -n 3 /var/log/sfdupes.log; else echo "a disk is not mounted; scan of /srv skipped"; fi
+30 3 * * * root /usr/local/bin/sfdupes scan /srv 2>>/var/log/sfdupes.log || tail -n 3 /var/log/sfdupes.log
 ```
 
 - The first scan creates `/var/lib/sfdupes/db.sqlite` and its directory. To keep
-  the database elsewhere, put `SFDUPES_DATABASE=/path/to/db.sqlite` directly
-  before `/usr/local/bin/sfdupes`.
+  the database elsewhere, put `SFDUPES_DATABASE=/path/to/db.sqlite` before the
+  command on the same line.
 - `scan` writes nothing to stdout. Its stderr, appended here to
   `/var/log/sfdupes.log`, holds:
     - one line naming the database;

TODO.md

@@ -29,9 +29,6 @@
 Newest first. An entry ends with its date and its issue or branch, where those
 are known.
 
-- `README.md` says what a disk that is not mounted does to the database. The
-  daily `cron` line runs `scan` only when `mountpoint` reports `/srv` as mounted
-  (2026-11-11, https://git.eeqj.de/sneak/sfdupes/issues/175)
 - `report` and `trees` leave out a copy that was deleted after the scan. Each
   checks only the copy itself with `lstat`: the file, or the directory at the
   top of a tree. A group left with one copy is not reported. A missing `scan`