Answers for "how to compress all branches on branch into one branch back to the master"

how to compress all branches on branch into one branch back to the master

git checkout yourBranch
 git reset $(git merge-base master yourBranch)
 git add -A
 git commit -m "one commit on yourBranch"
This isn't perfect as it implies you know from which branch "yourBranch" is coming from.
Note: finding that origin branch isn't easy/possible with Git (the visual way is often the easiest, as seen here).

EDIT: you will need to use git push --force