Add support for intermediate scripting steps to the forward-port process

Unlike the ink spots, some people do want to set the world on fire (symphony x, for instance), which nigh-guarantees every forward port from before to after the apocalypse will conflict and have to be hand-rolled.

Sometimes the apocalypse can be automated tho, and in those cases there might be cake it might be possible to fix-up the 'port: if a 'port needs to cross over the conversion commit:

• stop on October 22, 2077
• apply the first commit
• apply the conversion script
• ???
• have a first commit on October 24, 2077
• repeat for the first two commits (since we need the adapted working copy), then 3, 4, ...

Once we've rebuilt all the commits in the wasteland, resume the forwardporting process, possibly stopping at new apocalypses (e.g. butlerian jihad, cybernetic revolt, machine war, ..). along the way.

Start by locating the forward-port process and the conversion script it invokes; the issue names no files or tests. Define how intermediate scripting steps rebuild commits across a conversion commit and how the process resumes afterward, then add coverage for the described sequence and any later conversion points.