Improve example linter to not lint deleted paths
Improve example linter to not lint deleted paths

Summary:
It's easy to forget to do this when writing a new lint engine (like I recently
just did), so I added it to the example to improve documentation on how to write
a lint engine. This code is copied from PhutilLintEngine (as well as many
others).

Test Plan: php -l

Reviewers: jungejason, vrana, epriestley

Reviewed By: epriestley

CC: aran, Koolvin

Differential Revision: https://secure.phabricator.com/D2553

wxstars/arcanist,dguo-coursera/arcanist,fivestars/arcanist,hach-que/arcanist,aik099/arcanist,Symplicity/arcanist,fivestars/arcanist,memsql/arcanist,dguo-coursera/arcanist,kevinseelbach/arcanist,coursera/arcanist,haskell-infra/arcanist,calblueprint/arcanist,zhihu/arcanist,calblueprint/arcanist,wikimedia/phabricator-arcanist,phacility/arcanist,r4nt/arcanist,hach-que/arcanist,r4nt/arcanist,wharris/arcanist,wharris/arcanist,r4nt/arcanist,chundongwang/arcanist,calblueprint/arcanist,sneakpeeq/arcanist,wikimedia/phabricator-arcanist,wxstars/arcanist,eSpark/arcanist,memsql/arcanist,phacility/arcanist,codelabs-ch/arcanist,r4nt/arcanist,memsql/arcanist,phacility/arcanist,codelabs-ch/arcanist,julkiewicz/arcanist,eSpark/arcanist,Khan/arcanist,folsom-labs/arcanist,eSpark/arcanist,hach-que/arcanist,wikimedia/phabricator-arcanist,dguo-coursera/arcanist,memsql/arcanist,kevinseelbach/arcanist,jra3/arcanist,jra3/arcanist,KieranHayes/arcanist,haskell-infra/arcanist,wikimedia/phabricator-arcanist,alexmv/arcanist,aik099/arcanist,alexmv/arcanist,sneakpeeq/arcanist,PageUpPeopleOrg/arcanist-patches,julkiewicz/arcanist,folsom-labs/arcanist,coursera/arcanist,julkiewicz/arcanist,dguo-coursera/arcanist,kevinseelbach/arcanist,phacility/arcanist,calblueprint/arcanist,uber/arcanist,fivestars/arcanist,PageUpPeopleOrg/arcanist-patches,folsom-labs/arcanist,sneakpeeq/arcanist,codelabs-ch/arcanist,folsom-labs/arcanist,zhihu/arcanist,chundongwang/arcanist,KieranHayes/arcanist,alexmv/arcanist,codelabs-ch/arcanist,Symplicity/arcanist,aik099/arcanist,jra3/arcanist,zhihu/arcanist,sneakpeeq/arcanist,haskell-infra/arcanist,haskell-infra/arcanist,PageUpPeopleOrg/arcanist-patches,coursera/arcanist,zhihu/arcanist,Khan/arcanist,codelabs-ch/arcanist,kevinseelbach/arcanist,Khan/arcanist,uber/arcanist,phacility/arcanist,PageUpPeopleOrg/arcanist-patches,julkiewicz/arcanist,Symplicity/arcanist,KieranHayes/arcanist,jra3/arcanist,wharris/arcanist,aik099/arcanist,KieranHayes/arcanist,Khan/arcanist,chundongwang/arcanist,alexmv/arcanist,wxstars/arcanist,uber/arcanist,fivestars/arcanist,eSpark/arcanist,uber/arcanist,chundongwang/arcanist,coursera/arcanist,wxstars/arcanist,hach-que/arcanist,Symplicity/arcanist,wharris/arcanist