git create commit from diff between two branches

If you have two branches:

1. has-changes
2. needs-changes

And you want to move the changes from has-changes onto needs-changes, then do the following:

git checkout -b deleteme has-changes # Create temporary branch to build commit on
git reset --soft needs-changes       # Move diff into index
git commit                           # Create the diff patch commit
git checkout needs-changes           # Switch to branch that needs changes
git cherry-pick deleteme             # Apply the diff to the needs-changes
git branch -D deleteme               # Delete the temporary branch

It all comes down to a git reset --soft branch_b on top of a temp branch based on branch_a, and the result committed back to branch_b.

This is a step-by-step walking through the process:

#Start out on the branch with the code we want
git checkout branch_a

#create tmp branch same as branch_a (so that we don't change our local branch_a state during the operation)
git branch tmp

#working directory has all the code that we want, on tmp branch
git checkout tmp

# Change the branch head to the branch we want to be on. All the delta
# between the current source code and branch_b is now staged for commit
git reset --soft branch_b

# Move away from tmp, so our commit will go directly to branch_b
git checkout branch_b

# Now you can examine the proposed commit
git status

# Add the delta we want to the branch
git commit

# Sanity check that the branches have the same content now (should return an empty line)
git diff branch_A..branch_b

# Remove tmp, we don't need it anymore
git branch -D tmp

A simple way to make "the diff from branch_b..branch_a" into a commit is:

1. create and checkout branch tmp at branch_a (git branch tmp branch_a && git checkout tmp) (or git reset --hard branch_a on an existing branch)
2. git reset --soft branch_b
3. git commit

that commit will include all the diff between branch_b and branch_a.

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This works because

• 1. causes the files to reflect branch_a. This is the "end result" you want for the branch
• 2. “resets the head to branch_b” but “leaves all your changed files [i.e. branch_a head] as "Changes to be committed", as git status would put it.” ←(git reset --soft docs, with this example's branches added)