Clean up devel and main history

[spossann]

The history of devel (and thus main) contains some unwanted commits that were not squashed. This happened because the squashing was not automatic thus far in our new Github repo (although there are some older unwanted commits too).

We could thus clean devel using interactive rebase (git rebase -i) and then force push to devel. This will change the history of devel, which will lead to the following problems:

1. active feature branches cannot just rebase or merge devel, because the history has been rewritten. A more careful approach is necessary to bring feature branches up-to-the-new-date:
   https://stackoverflow.com/questions/67854979/how-to-work-around-force-pushing-of-an-upstream-branch

2. The history of main contains the commits of the releases. main is basically a copy of devel with some additional commits. Indeed, we always perform git merge main into a copy of devel and merge the ensuing branch into main (with additional commits). Hence main is like the upstream of devel, thus maybe a less complicated situation than point 1.

3. Releases point to specific commits. Will these be affected by a change in the history?

[spossann]

Firs thing I did was to just allow squash merging: https://github.com/struphy-hub/struphy/settings
This should avoid that we add unwanted commits in the future. If we keep more options tagged, one has to choose from a drop-down on the merge button I guess, which is error prone.

[spossann]

The command we can use to preserve merge commits during rebase is git rebase -i <SHA-1> --rebase-merges

[spossann]

We ended up redoing all the PRs since the Github transition manually. Done in the right sequence, this worked very well, with squash and merge. There is now a cleaner history in devel-clean, which will eventually become the default branch.