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

README.md

@@ -428,6 +428,15 @@ first	dupe	size
 /srv/a/big.iso	/srv/c/big-copy2.iso	4294967296
 ```
 
+Paths are raw bytes and may hold any byte except NUL, so the path
+columns (`first` and `dupe`) are escaped to keep every row one line of
+tab-separated fields: a backslash is written as `\\`, a tab as `\t`, a
+newline as `\n`, and a carriage return as `\r`. Every other byte is
+written unchanged, including bytes that are not valid UTF-8. Undoing
+those four escapes gives back the stored path. Grouping and ordering
+use the stored path, not the escaped one. The warnings `scan` prints on
+stderr are escaped the same way, so each warning is one line.
+
 Summary to stderr: records read, number of duplicate groups, number of
 dupe files, and total reclaimable bytes (sum of `size` over all dupe
 rows) in human units.
@@ -499,6 +508,9 @@ first	dupe	files	size
 /srv/a/project	/srv/backup/project	3417	104857600
 ```
 
+The `first` and `dupe` paths are escaped as described under "Report
+output format". The root directory's path is `/`.
+
 Summary to stderr: records read, number of duplicate-tree groups,
 number of dupe trees, and total reclaimable bytes (sum of `size` over
 all dupe rows) in human units.