Why `git am -3` apply patches that don't normally apply?

Hi,

From what I can see w/o `-3` `git am` follows the `patch`'es behavior. And the `patch`'es behavior is if there are lines in a patch that doesn't match the lines in the source file, it fails. For example, a source file:

11 2 3

A patch:

 1
 2
-3
+33

But `git am -3` will apply such a patch. That is, `patch` sees that the first line changed and that prevents it from applying the patch, but `git am -3` decides that it's okay. Why is that?

A script that reproduces the issue:

    set -eux

    mkdir a
    (cd a
    git init
    echo '1
    2
    3' > a
    git add a
    git commit -m 1,2,3
    sed -Ei 's/3/33/' a
    git add a
    git commit -m '3 -> 33'
    git format-patch -1 HEAD)

    mkdir b
    (cd b
    git init
    echo '11
    2
    3' > a
    git add a
    git commit -m 11,2,3

    git remote add a ../a
    git fetch a

    cat a
    cat ../a/0001-3-33.patch
    git --no-pager log --oneline --graph --all
    git am "$@" ../a/0001-3-33.patch  # try it w/ and w/o -3
    git --no-pager log --oneline --graph --all)

A couple of links, just in case, which have more information and show where it comes from:

https://stackoverflow.com/a/25847019/52499 https://gist.github.com/x-yuri/9907d5b9cbf29e84d510902a776741df https://gist.github.com/x-yuri/57d08f6afb10a89856d6e6a62abe30d4

Regards, Yuri