The "Generate slices" job spent ~90% of its wall time in actions/checkout
(4-27s across the last 10 runs, of a 10-32s job) pulling a ~212MB working
tree. Everything in the test matrix waits on it.
It doesn't need those files. `--generate-slices` returns before
`_approximately_count_tests`, so it only ever uses test file *paths* plus
the cached durations — it never opens a test file.
The blocker was discovery: `_discover_files` rglobs the filesystem, which
finds nothing under a sparse checkout. So add `--discover-from-git`, which
lists paths via `git ls-files`. Sparse checkout only clears the worktree
(entries are marked skip-worktree), so the index still carries every path
and enumerates exactly the same set. Skip-part filtering and the
root-override rule are duplicated to match `_discover_files` semantics.
Measured on a real clone of this repo:
full depth=1 clone: 7s 212M
blobless+sparse clone: 3s 3.4M (still sees all 2510 test paths)
Both discovery paths produce byte-identical slice JSON over the full 2472
test files, so slice assignment is unchanged.
Tests assert the properties that make this safe: the two discovery paths
agree on a full checkout, the git path still works when the files are
absent from disk, and the skip-part override behaves the same either way.
4e3768ea36