If the sentence still makes sense when ", dammit!" is appended to it, it's in the imperative mood and you are the subject.

Fair enough, but how does a command apply to a git commit? If it's telling the git commit what to do, that doesn't make sense, since the commit comment is supposed to be about the commit, not a command to the commit.

If it's telling me what to do, that doesn't make any sense either, since the work's already been done; the results of doing that work is the commit.

I guess it could be considered to be an abstraction of what request initiated the change, but what value does that provide when going back through git logs (when compared to a past or present verb tense that describes the changes)?