A few changes had cropped up with `_NIX_TEST_ACCEPT=1`:
1. Blake hashing test JSON had a different indentation
2. Store URI had improper non-quoted spaces
(1) was is just fixed, as we trust nlohmann JSON to parse JSON
correctly, regardless of whitespace.
For (2), the existing URL was made a read-only test, since we very much
wish to continue parsing such invalid URLs directly. And then the
original read/write test was updated to properly percent-encode the
space, as the normal form should be.
SHA-256 is Git's next hash algorithm. The world is still basically stuck
on SHA-1 with git, but shouldn't be. We can at least do our part to get
ready.
On the C++ implementation side, only a little bit of generalization was
needed, and that was fairly straight-forward. The tests (unit and
system) were actually bigger, and care was taken to make sure they were
all cover both algorithms equally.
