How to make git ignore a directory while merging

Question

As interesting and useful as the answer to How to setup a git driver to ignore a folder on merge was, it did not answer the original question which still has me stumped.

Answer 1

Let's say I'm on branch staging want to merge dev and ignore changes in /build folder:

git checkout staging
# go to staging branch

git checkout dev .
# this checkout dev file changes into staging

git reset HEAD build
# this remove added file in build folder

git clean -f
# this drops untracked files we just reseted

git checkout -- .
# this drops changed files we just rested

git status
# check is everything ok

git commit -am 'merge dev into branch'
# create a commit

git merge -s ours dev
# this create a fake merge to trick Git into thinking that a branch is already merged when doing a merge later on.

this solution is not perfect because it will merge all commits into one. which means cherry-pick some commits may not suitable.

but it's able to avoid lots of commit in build folder caused by rename/new/delete that need to clean up.