Make nix why-depends quieter by default

Unless `--precise` is passed, make `nix why-depends` only show the
dependencies between the store paths, without introspecting them to
find the actual references.

This also makes it ~3x faster

tests/dependencies.builder0.sh

@@ -4,7 +4,7 @@
 mkdir $out
 echo $(cat $input1/foo)$(cat $input2/bar) > $out/foobar
 
-ln -s $input2 $out/input-2
+ln -s $input2 $out/reference-to-input-2
 
 # Self-reference.
 ln -s $out $out/self

tests/gc.sh

@@ -18,7 +18,7 @@ if nix-store --gc --print-dead | grep -E $outPath$; then false; fi
 
 nix-store --gc --print-dead
 
-inUse=$(readLink $outPath/input-2)
+inUse=$(readLink $outPath/reference-to-input-2)
 if nix-store --delete $inUse; then false; fi
 test -e $inUse
 
@@ -35,7 +35,7 @@ nix-collect-garbage
 
 # Check that the root and its dependencies haven't been deleted.
 cat $outPath/foobar
-cat $outPath/input-2/bar
+cat $outPath/reference-to-input-2/bar
 
 # Check that the derivation has been GC'd.
 if test -e $drvPath; then false; fi

tests/why-depends.sh

@@ -9,5 +9,13 @@ cd $TEST_HOME
 nix-build ./dependencies.nix -A input0_drv -o dep
 nix-build ./dependencies.nix -o toplevel
 
-nix why-depends ./toplevel ./dep |
-    grep input-2
+FAST_WHY_DEPENDS_OUTPUT=$(nix why-depends ./toplevel ./dep)
+PRECISE_WHY_DEPENDS_OUTPUT=$(nix why-depends ./toplevel ./dep --precise)
+
+# Both outputs should show that `input-2` is in the dependency chain
+echo "$FAST_WHY_DEPENDS_OUTPUT" | grep -q input-2
+echo "$PRECISE_WHY_DEPENDS_OUTPUT" | grep -q input-2
+
+# But only the “precise” one should refere to `reference-to-input-2`
+echo "$FAST_WHY_DEPENDS_OUTPUT" | (! grep -q reference-to-input-2)
+echo "$PRECISE_WHY_DEPENDS_OUTPUT" | grep -q reference-to-input-2