[MERGE] fix bzr log --short and add a test

[Ian Clatworthy]

John Arbash Meinel wrote:

> c) Remove the special casing of logging the entire mainline. In my
> timing tests for bzr.dev, there was no difference between "bzr log
> --short" with or without that code path, and it is one less bit of code
> that needs to be maintained and tested.

Beware of just looking at the total time. This code path is there
because it provides a feature, namely incremental generation of output.
(Note that linear_view_revisions is an iterator while the standard code
path collects the result in a list before displaying anything.)

FWIW, a primary feature of my refactoring patch is to incrementally
produce output as the rule, not the exception, when --short or --line
are used.

Ian C.