You can use git show referencing the third parent from your current commit (i.e. the second ancestor from HEAD). Also, git show accepts the same format string as git log:
git show HEAD~2 --pretty=format:"%h" --no-patch
Update (2016-12-01)
An even better way would be to use the rev-parse plumbing command with the --short option to output the abbreviated (7 characters) commit SHA-1:
git rev-parse --short HEAD~2
Or you could also specify the exact length of the commit SHA-1:
git rev-parse --short=4 HEAD~2