git show only merge commits
git log --merges
git see all commits without merge commits
git log --no-merges
git log show only merge commits
git log --merges --first-parent master \ --pretty=format:"%h %<(10,trunc)%aN %C(white)%<(15)%ar%Creset %C(red bold)%<(15)%D%Creset %s"
Source: stackoverflow.com