fix(git-checkout): recover in narrowed-refspec (single-branch) workspaces

[fdaviddpt]

Problem

git-checkout:<branch> fails with ref '<branch>' not found even after fetch in workspaces whose remote.origin.fetch is narrowed to a single branch (e.g. +refs/heads/master:refs/remotes/origin/master). The auto-fetch fallback runs git fetch --all --prune, which only updates FETCH_HEAD — it never creates refs/remotes/origin/<branch> — so the retry git checkout <branch> still fails.

Hit in DVSI Kevin-style single-branch automation worktrees.

Fix

When the retry still hits a pathspec error, fall back to the manual workaround:

git fetch origin <ref>
git checkout -B <ref> FETCH_HEAD

-B is safe here: this block only runs after the first plain git checkout <ref> already failed, so a pre-existing local branch (which would have made that succeed) is impossible — -B only ever creates the branch.

Audit

git-diverge and git-merge do not share the bug — neither auto-fetches nor assumes origin/* tracking refs; both verify a local ref and tell the user to fetch.

Tests

Added test_narrowed_refspec_recovers_via_single_ref_fetch. 5/5 pass.

Closes #267

[fdaviddpt]

Rebased onto current master and dropped the bundled #241 byte-cap commits — those were already merged separately, so this PR now contains only the #267 git-checkout fix (checkout.py +13, test +35). Also chained the narrowed-refspec fallback after the #277 remote-only-branch path that landed in master since this branch was cut, so both recovery cases are handled: #277 creates a tracking branch when a remote ref exists, #267 fetches + resets to FETCH_HEAD when none does. 5/5 checkout tests pass.