match: make exactmatcher.visitchildrenset return file children as well
Previously, if we had an exactmatcher like ['foo.txt', 'a/bar.txt', 'a/b/c/baz.txt'], we'd
get back the following data:
'.': {'a'}
'a': {'b'}
'a/b': {'c'}
'a/b/c': 'this'
'a/b/c/d': set()
This was incorrect, since visitchildrenset explicitly says not to pay attention
to 'foo.txt' and 'a/bar.txt' by not returning them or 'this'. Given the near
impossibility of making visitchildrenset reliabbly produce only subdirectories,
a previous commit has made it documented and expected that visitchildrenset can
return a set containing both files and subdirectories to visit, instead of
implying/requiring that visitchildrenset() return 'this' if there are files to
visit. This makes the code for exactmatcher match this clarified documentation.
Differential Revision: https://phab.mercurial-scm.org/D4365