Fixes graph scope mode showing no commits for old branches

[ianhattendorf]

Summary

Scoping to an old local branch in the Commit Graph (via the focus picker, sidebar select, or overview-card click) sometimes rendered an empty filtered view: no commits visible despite the scope being active. Three issues at the same fault line, fixed together:

• computeScopeAnchor collapses to a focal-only anchor when target === focal. The reflog-derived "base branch" for branches made via git branch X origin/Y resolves to the branch's own upstream, which equals the focal tip. The original guard returned {focalBranchTipSha} only, so the bare scope never got a real merge base. Fix: iterate the candidate chain (mergeTargetBranch → baseBranch → defaultBranch) and skip candidates that collapse to the focal tip, so defaultBranch (typically main) becomes the recovered merge target.
• applyAnchorToPendingScope dropped focalBranchTipSha on the initial-resolve path. Only patchScopeAnchor ever set it on the live scope, so _pendingFocalTipBranchRef in graph-app — which drains on this field to load the focal tip — never fired for newly-scoped branches. Fix: apply focalBranchTipSha whenever the anchor carries one, even if the merge base is unusable.
• onEnsureRowRequest only paged once. For shas deep in history (an old branch's tip or its merge base), a single git log --skip=N --max-count=500 call hit the getCommitsForGraphCore 10× defensive cap before reaching the target. Fix: pass limit=0 to engage getCommitsForGraphCore's existing "no count cap, break on found" mode — one streaming git log --all invocation, O(N) walk, single process spawn instead of iteration.

Plus the stash + retry machinery (_pendingAnchorForMergeBaseLoad, tryUpgradeScopeFromPendingAnchor, requestEnsureRowForMergeBase) to handle graph-app's parallel EnsureRowRequest for the focal tip racing/cancelling our merge-base request — re-issues once when rows arrive without the merge base, gives up cleanly when rows haven't grown across attempts.

Verification

Live repro in ~/dev/repos/vscode-gitlens against three branches:

Branch	Before	After
old-feature-combined-diff (local, 2020-04 tip)	0 visible rows	24 visible rows
old-feature-timeline (local, 2020-12 tip)	0 visible rows	19 visible rows
feature/combined-diff (remote-only)	0 visible rows	n/a — popover only surfaces local branches in real UI

Cold scope-to-deep-branch: ~3.9s to render (single git log walk to the 2020-era merge base, ~9,800 rows fetched). Subsequent scopes that reuse loaded rows: ~250ms.

Known follow-ups (not in this PR)

• The deep walk fetches every commit between HEAD and the merge base. For a 6-year-old branch that can be 16,000+ commits + a 25s --shortstat stats follow-up. A targeted slab fetch (git log mergeBase..branchRef) would load ~50–100 commits instead — see review thread for a sketch.
• getBaseBranchFromReflog in packages/git-cli/src/providers/branches.ts still returns the branch's own upstream as "base" for git branch X origin/Y-style branches. This PR neutralizes the bad value for graph scope, but it still flows to branch-tree comparison defaults, the merge-target picker, and overview enrichment. Worth a separate ticket.

Test plan

• Scope to a freshly-created local branch tracking a recent remote — commits render
• Scope to a several-year-old local branch — commits render (within ~5s on a large repo)
• Scope to the current/default branch — same behavior as before (no merge-target overlay; all ancestors visible)
• Scope, then clear scope, then re-scope to a different branch — no stale anchors / no retry storms
• Refs invalidation (rebase, branch delete) mid-scope — stash cleared correctly