Still using `partial_fill` when creating FileTreeQuery

There were some subtle bugs in the use of get_all, which should now be fixed. They were caused by having multiple optional variables and the result not being deterministic if these were iterated in different orders.

which do not vary, and only match zero or one file.

more sense, and to make it easier to un-ambiguouosly identify the location
of a Match in a filetree hierarchy

default to returning a flat list of matches. subtree-related fixes to scan
function

operators to Match class so it can be ordered. Protect Match attributes via
@property

mutate canonical version

for which an optional variable is not present