Match/ChildMatch accept a ** pattern which is not noted in the doc
string, nor do any of the docs or tests specify whether the match is
greedy (i.e., can 'foo/**/bar' match paths with additional intermediate
bar directories?).
Add a note to the doc string and add test cases for greedy matches.
This improves restore performance by several orders of magniture by not
going through the whole tree recursively when we can anticipate that no
match will ever occur.
