git create commit from diff between two branches

Question

I have two branches which have very little similar history, but are related to each other.

I want the changes between those two in one git commit.

Answer 1

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