Escape tab, newline, CR and backslash in report paths (closes #7)

A path holding a tab or newline split a row of the report or trees
output. The path columns of both now write a backslash, tab, newline
and carriage return as \\, \t, \n and \r; every other byte is written
unchanged. Grouping and sorting still use the stored path. Warnings
on stderr are escaped the same way in warnf, so each stays one line.

In trees, the root directory's node now has the path "/" instead of
an empty string, and its children's paths start with a single slash.

README states the rule under "Report output format".

Model: opus-5-5

trees.go

@@ -62,7 +62,8 @@ func runTrees(ctx context.Context) error {
 		first := g[0]
 		for _, n := range g[1:] {
 			_, err = fmt.Fprintf(out, "%s\t%s\t%d\t%d\n",
-				first.path, n.path, first.fileCount, first.totalSize)
+				escapePath(first.path), escapePath(n.path),
+				first.fileCount, first.totalSize)
 			if err != nil {
 				return fmt.Errorf("write stdout: %w", err)
 			}
@@ -87,8 +88,8 @@ func runTrees(ctx context.Context) error {
 
 // buildHierarchy reconstructs the directory hierarchy from the record
 // paths under a synthetic super-root. Paths are split on "/"; for
-// absolute paths the first component is empty, which simply becomes a
-// top-level node representing "/". It returns the super-root and every
+// absolute paths the first component is empty, which becomes the
+// top-level node with path "/". It returns the super-root and every
 // directory node created.
 func buildHierarchy(recs []scanRec) (*treeNode, []*treeNode) {
 	super := &treeNode{}
@@ -102,9 +103,17 @@ func buildHierarchy(recs []scanRec) (*treeNode, []*treeNode) {
 		for _, c := range comps[:len(comps)-1] {
 			child := node.dirs[c]
 			if child == nil {
-				childPath := c
-				if node != super {
-					childPath = node.path + "/" + c
+				childPath := node.path + "/" + c
+
+				// The root directory's path is "/", not empty, and its
+				// children's paths start with one slash, not two.
+				switch {
+				case node == super && c == "":
+					childPath = "/"
+				case node == super:
+					childPath = c
+				case node.path == "/":
+					childPath = "/" + c
 				}
 
 				child = &treeNode{path: childPath, parent: node}