Pushed.

Yes, sounds good (no experiments needed).

I assume we do not need experiments, do we? If not, I'll merge this issue and
re-create a pull request for issue668.

Looks good, thanks!

I am done making the discussed changes; please see merge_and_shrink_heuristic.cc
and shrink_strategy.* (or the single commits).

I answered a few comments and agree with the rest (did not push yet so that the
comments are still easily visible in the pull request).

I'm done making comments on bitbucket.

I added a VerboseLevel option. The change affects many files, but it is still
fairly simple: https://bitbucket.org/SilvanS/fd-dev/pull-requests/21/issue670/diff

As discussed in issue668, we want to make the change that adds a verbose option
"all over the place" in merge-and-shrink in a separate issue, to ease reviewing.

More precisely, I thought of having a general verbose option for the heuristic,
but not for all other components. All other components will have a verbose
parameter for their main methods to enable or disable producing logging output.