Undo a fast-forward merge

If you reset the branch master, it won't touch develop branch. In order to reput all in order, you should do:

git checkout master
git reset --hard sha-of-B
git checkout develop
git reset --hard sha-of-D
git checkout master
git merge develop --no-ff

imagine you are still on master where you merged and pushed
git reset --hard @{1}
- this resets branch “master” to where it was one step back on your computer (i.e. at “B”)
for develop do nothing, because it should still be at “D”
git push publish develop
- this pushes branch “develop” in the usual way
git push publish master --force-with-lease
- pushes branch “master” backwards, with the permission gained via --force-with-lease in a safe way
- the use of --force might silently overwrite work from others, so I never use that
or both push operations together with this surprising syntax:
- git push publish develop --force-with-lease=master

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