make diff git as commit

$ git diff HEAD^^ HEAD main.c
$ # show diff between HEAD(current commit) and HEAD^^(two commits back) for main.c

git diff --name-only SHA1 SHA2

A simple way to do it is:

create and checkout branch tmp at branch_a (git branch tmp branch_a && git checkout tmp)
git reset --soft branch_b
git commit
that commit must have all the diff