Merge topic 'cleanup-RunSingleCommand'

1040e690 cmSystemTools: Teach RunSingleCommand to merge child pipes when possible
ce3b713b cmSystemTools: Simplify RunSingleCommand output string construction
dc039cc0 cmSystemTools: Drop redundant condition in RunSingleCommand
ffa2a8c9 cmSystemTools: Rename OUTPUT_NORMAL to OUTPUT_FORWARD to clarify its purpose
92e9bb21 cmcmd.cxx: Remove unused code in __run_iwyu implementation
fb1526f5 cmake: Change `-E chdir` to pass through stdout/stderr directly