"bzr log" currently spews all its output to standard output, even if
huge.  I think it would prove more user-friendly to automatically spawn
a pager for commands with large amounts of output, such as "bzr log", if
standard output refers to a terminal.

If standard output does not refer to a terminal, these commands could
continue to emit all their output as they currently do, keeping
compatibility with any use of these commands in a pipeline.

- Josh Triplett