## Why
Git can interpret a requested commit SHA as a branch name when the remote's
default branch has the same name. This can cause a marketplace plugin source
to materialize a different commit than the one it pinned.
## What changed
Resolve `HEAD` after checking out a SHA-pinned Git plugin source and reject the
source when the resolved commit does not exactly match the requested SHA.
Ref-name checkouts retain their existing behavior.
## Testing
Add a regression test with a default branch named after another commit's SHA
and verify that materialization rejects the mismatched checkout.
GitOrigin-RevId: c19cbd98ee6167dad7b2ee72e283b3bd30b59713
690995b7c1