git rebase after previous git merge

git merge --squash is now my preferred way of rebasing after a large amount of work and many merges (see this answer). If the branch you're working on is called my-branch and you want to rebase from main then just do the following:

git checkout my-branch
git branch -m my-branch-old       # rename from my-branch to my-brand-old
git checkout main
git checkout -b my-branch         # make new my-branch branch
git merge --squash my-branch-old  # squash and merge changes from old branch 
git commit

Two remarks:

• you can rebase your own (non yet pushed) work as many time as you want on top of newly fetched commits.
• You could avoid the merge conflicts (during rebase) if you had activated git rerere, which is done for this kind of situation.
  See more at git rerere.

You can take all of the changes in your branch and put them into a new commit in master with the following:

git diff master > my_branch.patch
git checkout master
patch -p1 < my_branch.patch

Then stage your files and commit.

Regarding the replay of merge conflicts, you can use git rerere to maintain a database of how merge conflicts have already been solved, so that performing a rebase that results in the same conflicts will have the laborious parts done for you automatically.

https://hackernoon.com/fix-conflicts-only-once-with-git-rerere-7d116b2cec67

git config --global rerere.enabled true

The one thing to look out for is that if you resolved something incorrectly it will be automatically borked for you next time too, and you may not really realize it.

More formal documentation here: https://git-scm.com/docs/git-rerere

I found @sky's answer helpful. And I made it into a squash-rebase function.

Assuming you are on your-branch, what this does is create a branch your-branch-tmp that is the result of doing merge --squash on main. It also preserves all the commit messages and lets you edit. Importantly, it also leaves your current branch alone.

After running squash-rebase, if you're satisfied with what it did, you must do a hard reset to the tmp branch. Then in effect you've completed a squash rebase.

function squash-rebase() {
  MESSAGES="$(git log $(git merge-base main HEAD)..HEAD --reverse --format=%B)"
  SRC_BRANCH="$(git_current_branch)"
  TMP_BRANCH="$(git_current_branch)-tmp"
  echo "creating temp branch $TMP_BRANCH from $(git_current_branch)"
  git checkout -b $TMP_BRANCH main  # create tmp branch from main and checkout
  git merge $SRC_BRANCH --squash
  git commit -m "$MESSAGES" -n
  git commit --amend -n # just so you can edit the messages
  git checkout $SRC_BRANCH
  echo "IMPORTANT: run git reset --hard $TMP_BRANCH to complete squash rebase."
  echo "First, though, you may run git diff $TMP_BRANCH to make sure no changes."
  echo "Finally, delete the tmp branch with git branch -D $TMP_BRANCH"
}

Note: assumes you are using oh-my-zsh, otherwise you may have to find another way to get current branch name.