test(git,encryption): anchor fixture roots at temp directories, not "/" - #398

Issue #392, the fixture half. Test fixtures built runtime environments on
fsroot.OpenWritableUnconfined("/"), a whole-filesystem root that cannot
address absolute Windows paths: filepath.Rel("/", "C:\...") cannot cross
volumes, so fsroot.makePath's assert fired and TestReceipt_RestoreEncoded
panicked on the windows leg.

The issue said four sites; the uncapped sweep found ten — the original grep
was head-capped, and the correction is recorded on the issue. All ten now
anchor at temp directories: encryption's receipt test and git's receipt test
root at t.TempDir() and join their fixture paths beneath it; git's
testActivation and newTestProvider take an explicit rootDir parameter with
every caller passing t.TempDir() and deriving path literals from it (the
hardcoded /tmp/clone-dest, /tmp/dest, /tmp/shallow, /tmp/checkout-repo, and
/tmp/pull-repo literals become filepath.Join forms, and the
derived-directory assertions follow the root instead of expecting "/repo");
newRes anchors at the path's own directory; the narrating and observe
provider fixtures root at t.TempDir().

One test needed more than mechanical anchoring:
TestDiscoverResource_WindowsShapedPath_IsAPath exists to prove a drive-colon
path is treated as a path, not a URI scheme. Its input now borrows the
fixture root's own volume on Windows — a foreign volume cannot be made
root-relative — while on Unix VolumeName is empty and the original literal
"D:" shape survives, so the scheme-collision subject is exercised on both
platforms.

Anchoring at temp directories also confines every fixture properly: the
tests could previously address the whole filesystem.

Expected windows-leg effect: the ~5-failure panic cluster clears. The writ
System-layer TargetRoot "/" design question is #392's remaining half,
deliberately untouched here.

Verified: make vet, make lint-all, full make test green — zero FAIL or panic
lines by count; gofmt clean.
Issue: #392