A cheap fingerprint of local branches and HEAD that future code can poll to
detect when refs have moved externally.
Branches come from a porcelain for-each-ref. HEAD is read directly from
.git/HEAD: that avoids spawning a child process and captures the symref-or-hash
distinction we need to tell "detached at X" apart from "on a branch pointing at
X" — they share a commit hash, which is exactly the situation at the end of a
rebase when HEAD reattaches to the branch. The reftable backend doesn't keep a
real .git/HEAD (it writes a fixed stub), so when we see that stub or the file is
unreadable we fall back to porcelain commands, which are backend-agnostic.
Uses DontLog so a future polling caller won't spam the command log. Not yet
wired up to any caller.
