How to see commits that were merged in to a merge commit?

git log abc123^..abc123
shows the commits that got merged into merge-commit abc123.

Create a git alias log-merge for easy reuse:

$ git config --global alias.log-merge \
'!f() { git log --stat "$1^..$1"; }; f'
$ git log-merge abc123

For a one-line version:

$ git config --global alias.log-merge-short \
'!f() { git log --pretty=oneline "$1^..$1"; }; f'