How do I reverse a specific hunk of a commit in git?

git checkout -p $REF -- path/to/file

e.g.,

git checkout -p HEAD^ myfile

Where $REF is a ref name or commit ID that specifies the commit you want to take the file state from. For example, to selectively revert changes made in the last commit, use HEAD^.

git difftool $REF -- /path/to/file

where $REF is a ref name or commit ID that specifies the commit you want to take the file state from. For example, to selectively revert changes made in the last commit, use HEAD^.

This question was already answered by @cdhowie, but I find it somewhat nicer to use an interactive difftool like meld to selectively restore old hunks/lines of code, especially if there is a newly-introduced, hard-to-find bug in the code.

To revert one or more hunks from specific commit(s) do like this - using add -p in essence:

git revert --no-commit <commit>…​ 
git reset                              # unstage things
git add -p [OPTIONS] [<pathspec>…​]     # choose hunks interactively
git restore .                          # wipe the rest (in root directory)
... maybe further changes ...
git commit [--fixup] ...

Future git versions may support git revert -p directly...

Note: Methods like git checkout/restore -p ... do not consistently revert hunks from a specific commit but go to pick parts from a certain file state - possibly loosing changes in other later commits.