$ git add --patch foo # add just first part to the index
$ git stash save --keep-index # save all other changes to the stash
$ edit/build/test first part
$ git commit foo -m 'First part' # commit fully tested change
$ git stash pop # prepare to work on all other changes
$ git add --patch foo # add just first part to the index
$ git stash save --keep-index # save all other changes to the stash
$ edit/build/test first part
$ git commit foo -m 'First part' # commit fully tested change
$ git stash pop # prepare to work on all other changes