[lit] Improve readability of failing scripts.
- This only applies to scripts executed by the _internal_ shell script interpreter.
- This patch reworks the log to look more like a shell transcript, and be less verbose (but in the interest of calling attention to the important parts).
Here is an example of the new format, for commands with/without failures and
$ true $ echo hi hi $ false note: command had no output on stdout or stderr error: command failed with exit status 1
[lit] Improve logging with file redirection.
- This will cause lit to automatically include the first 1K of data in redirected output files when a command fails (previously if the command failed, but the main point of the test was, say, a FileCheck later on, then the log wasn't helpful in showing why the command failed).