batches: make `-f` optional

Created by: LawnGnome

New behaviour (assuming COMMAND is any command that accepted -f, including apply, preview, repos, or validate):

• src batch COMMAND on a TTY: will wait for 250ms in case there's input on stdin; otherwise errors
• src batch COMMAND with a file redirected in: waits indefinitely for input
• src batch COMMAND -f -: waits indefinitely for input
• src batch COMMAND FILE: reads from FILE; equivalent to src batch COMMAND -f FILE
• src batch COMMAND -f FILE: works as before

In summary, this shouldn't break backward compatibility in any case except for the user slowly typing in a batch spec on stdin.

Fixes #680.