Hardened sha1 does detect this new attack. Easy to test: Check their pair of files into a git repo and see that they have different checksums, while sha1sum(1) generates the same for both.
$ mkdir sha1
$ cd sha1
$ curl -O https://sha-mbles.github.io/messageA
...
$ curl -O https://sha-mbles.github.io/messageB
...
$ echo foo > bar
$ echo foo > baz
$ openssl sha1 *
SHA1(bar)= f1d2d2f924e986ac86fdf7b36c94bcdf32beec15
SHA1(baz)= f1d2d2f924e986ac86fdf7b36c94bcdf32beec15
SHA1(messageA)= 8ac60ba76f1999a1ab70223f225aefdc78d4ddc0
SHA1(messageB)= 8ac60ba76f1999a1ab70223f225aefdc78d4ddc0
$ git init
Initialized empty Git repository in ...
$ git add *
$ git commit
[master (root-commit) b274c88] sha1 collision test
...
4 files changed, 2 insertions(+)
create mode 100644 bar
create mode 100644 baz
create mode 100644 messageA
create mode 100644 messageB
$ git ls-files -s *
100644 257cc5642cb1a054f08cc83f2d943e56fd3ebe99 0 bar
100644 257cc5642cb1a054f08cc83f2d943e56fd3ebe99 0 baz
100644 5a7c30e97646c66422abe0a9793a5fcb9f1cf8d6 0 messageA
100644 fe39178400a7ebeedca8ccfd0f3a64ceecdb9cda 0 messageB
$> git doesn't really use SHA-1 anymore, it uses Hardened-SHA-1 (they just so happen to produce the same outputs 99.99999999999...% of the time).[1]
https://stackoverflow.com/questions/10434326/hash-collision-...
There's essentially no chance that the string "foo\n" fell into that tiny probability of difference. The reason there's a difference is because before git hashes something, git will do various processing to it (maybe appending and prepending various things) and those things broke the carefully created collision. But a chosen-prefix attack might mean those various things can be accounted for, and a collision could still be found.
So we need to directly run hardened SHA1 on the data, which I believe is located at https://github.com/cr-marcstevens/sha1collisiondetection
As seen in https://github.com/git/git/blob/master/sha1dc_git.c
So I tested that one:
$ sha1collisiondetection-master/bin/sha1dcsum bar baz messageA messageB shattered-1.pdf shattered-2.pdf
f1d2d2f924e986ac86fdf7b36c94bcdf32beec15 bar
f1d2d2f924e986ac86fdf7b36c94bcdf32beec15 baz
4f3d9be4a472c4dae83c6314aa6c36a064c1fd14 *coll* messageA
9ed5d77a4f48be1dbf3e9e15650733eb850897f2 *coll* messageB
16e96b70000dd1e7c85b8368ee197754400e58ec *coll* shattered-1.pdf
e1761773e6a35916d99f891b77663e6405313587 *coll* shattered-2.pdf