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.

Model: opus-5-5

internal/sfdupes/report_test.go

@@ -491,9 +491,9 @@ func TestReadReportChecksOnlyPossibleDuplicates(t *testing.T) {
 
 	a, u, e := testHash("a"), testHash("u"), sha256.Sum256(nil)
 	recs := []scanRec{
-		{size: 9, head: a, tail: a, content: a, path: "/x/1"},
-		{size: 9, head: a, tail: a, content: a, path: "/x/2"},
-		{size: 9, head: a, tail: a, content: a, path: "/x/3"},
+		{size: 9, head: a, tail: a, content: a, path: "/copy/1"},
+		{size: 9, head: a, tail: a, content: a, path: "/copy/2"},
+		{size: 9, head: a, tail: a, content: a, path: "/copy/3"},
 		{size: 9, head: u, tail: u, content: u, path: "/lonely"},
 		{size: 0, head: e, tail: e, content: e, path: "/empty1"},
 		{size: 0, head: e, tail: e, content: e, path: "/empty2"},
@@ -521,7 +521,7 @@ func TestReadReportChecksOnlyPossibleDuplicates(t *testing.T) {
 
 	slices.Sort(checked)
 
-	want := []string{"/x/1", "/x/2", "/x/3"}
+	want := []string{"/copy/1", "/copy/2", "/copy/3"}
 	if !slices.Equal(checked, want) {
 		t.Errorf("paths checked on disk = %q, want %q", checked, want)
 	}

internal/sfdupes/trees_test.go

@@ -503,20 +503,16 @@ func TestCollectTreeGroupsDifferingParents(t *testing.T) {
 // TestCollectTreeGroupsChecksOnlyReportedCopies checks that trees asks
 // whether a directory is on disk only for the members of the groups it
 // returns. It never asks about /a/proj/sub, which lies inside a copy
-// and whose group the group of /a/proj implies, nor about /a/empty,
-// which holds 0 bytes.
+// and whose group the group of /a/proj implies.
 func TestCollectTreeGroupsChecksOnlyReportedCopies(t *testing.T) {
 	t.Parallel()
 
-	f, g, u, e := testHash("f"), testHash("g"), testHash("u"),
-		sha256.Sum256(nil)
+	f, g, u := testHash("f"), testHash("g"), testHash("u")
 	recs := []scanRec{
 		{size: 10, head: f, tail: f, content: f, path: "/a/proj/f"},
 		{size: 20, head: g, tail: g, content: g, path: "/a/proj/sub/g"},
-		{size: 0, head: e, tail: e, content: e, path: "/a/empty/.gitkeep"},
 		{size: 10, head: f, tail: f, content: f, path: "/b/proj/f"},
 		{size: 20, head: g, tail: g, content: g, path: "/b/proj/sub/g"},
-		{size: 0, head: e, tail: e, content: e, path: "/b/empty/.gitkeep"},
 		// /a lacks this file, so /a and /b are no copies of each other.
 		{size: 30, head: u, tail: u, content: u, path: "/b/lonely"},
 	}