adopt-project.py: execute counts a leg's commits with git rev-list --count HEAD and no --, so a leg that keeps a root file named HEAD stops execute with exit 128 after git filter-repo, before that leg is pushed #1215

_extract_leg counts a leg's kept commits with git rev-list --count HEAD in the leg's work clone, right after git filter-repo (adopt-project.py:13915 on main ef619c74). That argv has no --. When the leg tracks a file named HEAD at its root, the work tree holds that file, and git reads the argument both ways:

rev-list --count HEAD     exit=128  fatal: ambiguous argument 'HEAD': both revision and filename
rev-list --count HEAD --  exit=0
rev-parse HEAD            exit=0

Measured on git 2.43.0 by lane 1's reviewer r1180d (probe P6, in its scratch clone), while reviewing #1180, which fixed the same class of argv for the source-partial-clone walk.

What it costs:

• git_out raises, so execute stops after git filter-repo and before that leg is pushed.
• The line just above it reads head = git_out(["rev-parse", "HEAD"], cwd=work), so the id is already in hand.
• Not yet measured: what an earlier leg left pushed by then, and whether check or plan would have warned.

The fix:

• Count head, the id from the line above, and end the argv with --: ["rev-list", "--count", head, "--"].
• Add a row: a source whose code leg keeps a root file named HEAD gets through execute with both legs pushed.

Same shape elsewhere, not measured:

• Source.commit_count runs ["rev-list", "--count", self.commit] (adopt-project.py:2899). A file named as the commit's id would trip it.
• shape-doctor.py passes a commit id to rev-list with no -- at about lines 5864 and 6166.

Found by lane 1 (openRepoShape-1); see #1180's review r1180d.