Martin von Zweigbergk <martinvonz@google.com> [Thu, 18 May 2017 09:04:37 -0700] rev 32362
match: make _fileroots a @propertycache and rename it to _fileset
The files in the set are not necesserily roots of anything. Making it
a @propertycache will help towards extracting a base class for
matchers.