Mismatched binary detection behaviour on diffs

Hey 👋

I have a peculiar case that I've created a reproducible for. If you clone this repository https://github.com/FintanH/git2-binary-check and do cargo run, you'll get the following output:

Path: "bin/cat"
Is binary: false
Path: "bin/ls"
Is binary: false
Path: "bin/test"
Is binary: false

However, if you were to run git diff 373b3404371e359404392aff715d504293112c8b abdedcec89e3c48ac520e4a96c7ea39364316b9e, you'll get:

diff --git a/bin/cat b/bin/cat
new file mode 100755
index 0000000..0bbb14b
Binary files /dev/null and b/bin/cat differ
diff --git a/bin/ls b/bin/ls
new file mode 100755
index 0000000..87c2d51
Binary files /dev/null and b/bin/ls differ
diff --git a/bin/test b/bin/test
new file mode 100755
index 0000000..0cced79
Binary files /dev/null and b/bin/test differ

It seems like git2, and possibly libgit2, are not detecting binary files in the same manner as the git CLI is doing.

I checked on git2-0.17.0 and git2-018.1 and my git version is git version 2.40.1.

Let me know if I can do anything else to help track down this discrepancy.

Start by cloning the linked git2-binary-check repository and run cargo run, then compare its output with git diff 373b3404371e359404392aff715d504293112c8b abdedcec89e3c48ac520e4a96c7ea39364316b9e. Investigate the binary-detection behavior in git2/libgit2 using git2-0.17.0, git2-0.18.1, and Git 2.40.1; done means the discrepancy is explained and detection behavior is aligned or clearly documented.