Unexpected error message on case-sensitive file system when file is included in program both via relative and absolute names

[vladima]

// a.ts
<reference path="b.ts"/>

// b.ts
var x

vladima@vladima-OptiPlex-990:~/Sources/TypeScript$ node built/local/tsc.js a.ts $(pwd)/b.ts
error TS1149: File name '/home/vladima/Sources/TypeScript/b.ts' differs from already included file name 'b.ts' only in casing

[vladima]

fixed in #5275 via e811fec