Getting differences of file between specific revisions/branches

I have 2 different branches A,B that have a (slightly different) version of a file X. I am interested in getting the commits that added some specific patterns in branch B. What I do roughly: diff files| grep "^\+" | grep "$PATTERN" | for loop grep -n.. do git blame -L done This works but I was wondering if I am re-inventing/going a roundabout for something that is readily supported in git. Is there a better way?