Leave empty files out of report and trees (closes #178)

Removing an empty file frees nothing, yet every `.gitkeep`,
`__init__.py` and empty log formed one `report` group, and directories
holding only empty files were reported as 0-byte duplicate trees. Now
`report` writes no group of size 0 and `trees` writes no group whose
trees hold 0 bytes. An empty file still counts toward the digest of a
tree that holds other files. No option, as approved on the issue. The
README's definition-of-done test now expects `empty1` and `empty2`
nowhere.

Model: opus-5-5

README.md

@@ -665,6 +665,7 @@ Processing:
 - Records are grouped by signature, the four values
   `(size, head, tail, content)`.
 - Every group with two or more paths is a duplicate group.
+- Empty files form no group, because removing one frees nothing.
 - Within a group, paths are sorted in byte order. The first is `first`, and
   every other path is a `dupe`.
 - Groups are ordered by size, largest first, then by `first` path. The output is
@@ -713,7 +714,7 @@ The summary line on stderr gives:
 - the reclaimable bytes, the sum of `size` over all rows.
 
 ```
-report: 14 records read, 5 duplicate groups, 8 dupe files, 10.0 KiB reclaimable
+report: 14 records read, 4 duplicate groups, 7 dupe files, 10.0 KiB reclaimable
 ```
 
 ### `trees` mode
@@ -741,6 +742,9 @@ Processing:
 - `trees` builds the directory tree and computes every directory's digest. It
   then groups the directories by digest. Every group with two or more
   directories is a duplicate-tree group.
+- `trees` leaves out a group whose trees hold 0 bytes, because removing one
+  frees nothing. An empty file still counts toward the digest of a tree that
+  holds other files.
 - `trees` leaves out a group that a group of its members' parents already
   implies. A group is left out when its members' parents are distinct
   directories that all have the same digest. The parents' group, or an
@@ -1382,7 +1386,7 @@ All of the following, run in this directory, must pass:
     head -c 2000 /dev/urandom > "$d/a/unique.bin"     # same size, different content
     printf 'x' > "$d/tiny1"; printf 'x' > "$d/tiny2"  # 1-byte duplicates
     printf 'y' > "$d/tiny3"                           # 1-byte non-duplicate
-    : > "$d/empty1"; : > "$d/empty2"                  # empty duplicates
+    : > "$d/empty1"; : > "$d/empty2"                  # empty files
     # duplicate trees: t1 and t2 are identical; t3 differs by one filename
     mkdir -p "$d/t1/sub" "$d/t2/sub" "$d/t3/sub"
     head -c 3000 /dev/urandom > "$d/t1/f1"
@@ -1407,8 +1411,8 @@ All of the following, run in this directory, must pass:
       is the path that sorts first.
     - `t1/f1`, `t2/f1` and `t3/f1` form one group.
     - `t1/sub/f2`, `t2/sub/f2` and `t3/sub/f2renamed` form one group.
-    - `tiny1` and `tiny2` form a pair, and so do `empty1` and `empty2`.
-    - `unique.bin` and `tiny3` appear nowhere.
+    - `tiny1` and `tiny2` form a pair.
+    - `unique.bin`, `tiny3`, `empty1` and `empty2` appear nowhere.
     - The groups come in order of size, largest first.
 
     `trees` prints exactly one row, `$d/t1` as `first` and `$d/t2` as `dupe`.