Thank you for answering.
Also, I did check the dictionary before asked.
As others comments pointed out, it does have a meaning of adjacency or contrast, and I guessed that this is where this usage was coming from.
But still, even with this meaning, I feel a bit of an adversarial meaning.
"Contrast" itself is adversarial, right? Because it comes from comparison.
For instance, in "the chair against the wall", I can see the chair exerting force against the all, and the wall reacting to it with equal intensity and opposite direction (Newton's Third Law).
Yet, in Willson's tweet, I can't imagine anything like that coming off of a git repository and git notes, hence my comment.
As an addendum, my first language is Portuguese, and a direct translation for this word is "contra"¹ which fits all the meanings for "against" listed in Merriam-Webster, except the adjacency one. I've never seen "contra" being used for this meaning in the wild, and I guess this is the reason I feel a bit off about it and never used it myself for that meaning when communicating in English.