Initial state:
==============

A - B - C - D - E - F - G - H  master
             \
              \
               I - J - K - L  feature


SQUASH: After git checkout master; git merge --squash feature
==============================================================

A - B - C - D - E - F - G - H - N  master
             \
              \
               I - J - K - L  feature

N is a combination of I, J, K and L (they are squashed)

After git branch -D feature: 
============================

A - B - C - D - E - F - G - H - N  master


MERGE: After git checkout master; git merge feature:
====================================================

A - B - C - D - E - F - G - H - M  master
             \                /
              \              /
               I - J - K - L  feature

After git branch -D feature:
============================

A - B - C - D - E - F - G - H - M  master
             \                /
              \              /
               I - J - K - L

M is merge commit. From there you can look at I, J, K and L