Backport failing with "Patch format detection failed." in `git am`

[jjonescz]

Seen recently in several backports that don't have any conflicts. Example (https://github.com/dotnet/roslyn/actions/runs/19525506093/job/55897503610):

Verified jjonescz has write access to the repo.
/usr/bin/git ls-remote --exit-code --heads origin release/dev18.0
6c4a46a31302167b425d5e0a31ea83c9a9aa1d09	refs/heads/release/dev18.0
Backport target branch: release/dev18.0
Applying backport patch
/usr/bin/git checkout release/dev18.0
Switched to a new branch 'release/dev18.0'
branch 'release/dev18.0' set up to track 'origin/release/dev18.0'.
/usr/bin/git clean -xdff
/usr/bin/git config user.name github-actions
/usr/bin/git config user.email github-actions@github.com
/usr/bin/git checkout -b backport/pr-81341-to-release/dev18.0
Switched to a new branch 'backport/pr-81341-to-release/dev18.0'
/usr/bin/git ls-remote --exit-code --heads origin backport/pr-81341-to-release/dev18.0
/usr/bin/curl -sSL https://github.com/dotnet/roslyn/pull/81341.patch --output changes.patch
/usr/bin/git am --3way --empty=keep --ignore-whitespace --keep-non-patch changes.patch
Patch format detection failed.
Error: git am failed, most likely due to a merge conflict.

Doing an unauthenticated github request seems a bit suspicious, perhaps it's failing due to rate limits? Might help diagnosing this issue if the patch content file is written out into the logs.