Merge branch 'js/realpath-pathdup-fix' into maint

Git v2.12 was shipped with an embarrassing breakage where various operations that verify paths given from the user stopped dying when seeing an issue, and instead later triggering segfault. ... and then to down to 'maint'. * js/realpath-pathdup-fix: real_pathdup(): fix callsites that wanted it to die on error t1501: demonstrate NULL pointer access with invalid GIT_WORK_TREE