grep: reduce the cost of pathauditor checks when grepping working copy Running `time hg grep zxczxczxczxczxc -l` on mozilla-central: before: real 0m20,000s user 0m15,796s sys 0m4,189s after: real 0m10,903s user 0m8,964s sys 0m1,916s if vfs didn't call pathauditor at all: real 0m7,781s user 0m5,968s sys 0m1,790s Differential Revision: https://phab.mercurial-scm.org/D8582