Add toString and improve Javadoc of NotIgnoredFilter

Add toString and improve Javadoc of NotIgnoredFilter

Today while debugging some TreeWalk related code I noticed this
filter did not have a toString(), making it harder to see what the
filter graph was at a glance in the debugger.  Add a toString()
for debugging to match other TreeFilters, and clean up the Javadoc
slightly so its a bit more clear about the purpose of the filter.

While we are mucking about with some of this code, simplify
the logic of include so its shorter and thus faster to read.
The pattern now more closely matches that of SkipWorkTreeFilter.

Change-Id: Iad433a1fa6b395dc1acb455aca268b9ce2f1d41b
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>