git diff of a file between two commits

git diff <branch_hash> <other_branch_hash> -- <file_path>

-> git diff 0da94be  59ff30c > my.patch
-> git apply my.patch

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

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

$ git diff 012345..abcdef