Make fossil id extraction more robust for e.g. monster merge commits
Fix incorrect parent ordering of broken merge commit in dune-grid
Update filemaps for SD_REPLACE actions
Add modular framework for repository conversion

- Avoid duplication of more basic tasks like history rewriting
  and git repo creation

- Make reindendation optional by delegating to a nested script specified
  as an input argument
Port existing scripts to new framework
Avoid renaming branch mn-devel that will be deleted anyway
Fix two additional incorrectly reconstructed merges in dune-grid
Fix several faulty merges in dune-common
Add tests for conversion correctness

The script verify.sh reads in pairs of repository HEADS for a git
and a SVN repository (branch/tag names or commit SHA1 for git and
repository paths for SVN). It checks out those heads and runs diff
against the trees.

The *.heads files in this commit contain sensible default lists of
heads for the core modules.
Fix faulty merges in dune-geometry, dune-istl and dune-localfunctions