The idea would be, since you haven't pushed anything yet, to:
git stash
git reset --hard HEAD~
git commit --amend
That would allow you (in command line) to move the HEAD (resetting the index and the working tree, hence the importance of stashing any work in progress first), and edit the commit message.
You should be able to move HEAD from SourceTree (SRCTREE-770, supported since 1.3b3), but command-line can be more precise in this case.
Revert has created another commit which cancel the previous one: if you see your working tree is modified because of it, the reset --hard
is here to reset everything to your previous commit (the one without commit message).