[lit] Builtin cat does not read stdin for a "-" operand #240058

lit's builtin cat treats a - operand as a file name instead of standard input, unlike POSIX cat. A RUN line such as

# RUN: echo foo | cat - %t

fails with [Errno 2] No such file or directory: '-'.

Found with help from Claude Code. I reproduced and reviewed it myself.

Fix: #240060