nix/why-depends: fix output of `--precise`

I haven't checked when this was exactly introduced, but on Nix 2.16 I
realized that the additional lines inserted when using `--precise` are
completely separated from the tree:

    nix why-depends /nix/store/ccgr4faaxys39s091qridxg1947lggh4-evcxr-0.14.2 /nix/store/b7hvml0m3qmqraz1022fwvyyg6fc1vdy-gcc-12.2.0 --precise --extra-experimental-features nix-command
    /nix/store/ccgr4faaxys39s091qridxg1947lggh4-evcxr-0.14.2
        → /nix/store/lcf37pgp3rgww67v9x2990hbfwx96c1w-gcc-wrapper-12.2.0
            → /nix/store/b7hvml0m3qmqraz1022fwvyyg6fc1vdy-gcc-12.2.0
    └───bin/evcxr: …':'}.PATH=${PATH/':''/nix/store/lcf37pgp3rgww67v9x2990hbfwx96c1w-gcc-wrapper-12.2.0/bin'':'/':'}…
        └───bin/cpp: …k disable=SC2193.[[ "/nix/store/b7hvml0m3qmqraz1022fwvyyg6fc1vdy-gcc-12.2.0/bin/cpp" = *++ ]] &&…

This is apparently because `std::cout` is buffered and flushed in the
end whereas the rest of the output isn't. The fix is rather simple, just
use `logger->cout` as it's already the case for the rest of the code.

This way we also don't need to insert additional newlines in the `hits`
map since that's something the logger takes care of.

Also added a small test to make sure that the layout of this is somehow
tested to reduce the risk of further regressions here.

1 files changed, 5 insertions, 0 deletions

diff --git a/tests/why-depends.sh b/tests/why-depends.sh
index b35a0d1cf..9680bf80e 100644
--- a/tests/why-depends.sh
+++ b/tests/why-depends.sh
@@ -22,3 +22,8 @@ echo "$PRECISE_WHY_DEPENDS_OUTPUT" | grepQuiet input-2
 # But only the “precise” one should refer to `reference-to-input-2`
 echo "$FAST_WHY_DEPENDS_OUTPUT" | grepQuietInverse reference-to-input-2
 echo "$PRECISE_WHY_DEPENDS_OUTPUT" | grepQuiet reference-to-input-2
+
+<<<"$PRECISE_WHY_DEPENDS_OUTPUT" sed -n '2p' | grepQuiet "└───reference-to-input-2 -> "
+<<<"$PRECISE_WHY_DEPENDS_OUTPUT" sed -n '3p' | grep "    →" | grepQuiet "dependencies-input-2"
+<<<"$PRECISE_WHY_DEPENDS_OUTPUT" sed -n '4p' | grepQuiet "    └───input0: …"                          # in input-2, file input0
+<<<"$PRECISE_WHY_DEPENDS_OUTPUT" sed -n '5p' | grep "        →" | grepQuiet "dependencies-input-0"    # is dependencies-input-0 referenced