My first commit messages were in past tense, but slowly I moved to imperative. I was greatly influenced by Chris Beam's article How to write a Git commit message and I just can't recommend it enough. I will share a passage of the article:
The imperative can sound a little rude; that’s why we don’t often use it. But it’s perfect for Git commit subject lines. One reason for this is that Git itself uses the imperative whenever it creates a commit on your behalf.
That being said, I still find the past tense acceptable for commit messages. At the end, it's the message that is most important, while the words are just the means by which you express it.