Implement PhutilArrayWithDefaultValue to simplify building a map with a count or mask
Summary: This can greatly simplify some of the code in my getWorkingCopyStatus() changes for D4049. See comments for details.
Test Plan: Ran unit tests.
Reviewers: vrana
Reviewed By: vrana
CC: aran
Differential Revision: https://secure.phabricator.com/D4092