Understanding git merge --no-ff

git.sh

git init
echo a > a.txt
git add a.txt && git commit -m "a"

git checkout -b feature-b
echo b > b.txt
git add b.txt && git commit -m "b"

git checkout master
# ($)

### Two cases possible:

# 1) Plain merge (will fast forward in our case)
git merge feature-b
git log
## <hash> b
## <hash> a
# No merge commit in our case (because fast forward)

# 2) No-ff Merge (will not fast forward even if it's possible (in our case it was possible))
git merge --no-ff feature-b
git log
## <hash> Merge commit
## <hash> b
## <hash> a

# Going back to git checkout master ($)
# Let's commit some work on master
echo c > c.txt
git add c.txt && git commit -m "c"

git merge feature-b
# Now fast forward is not possible
git log
## <hash> Merge commit
## <hash> c
## <hash> b
## <hash> a