Accept and discard fragments in getFlakeRefForCompletion

Otherwise trying to complete `nix build foo#bar --update-input <Tab>`
fails with "unexpected fragment"

src/libcmd/command.hh

@@ -134,7 +134,7 @@ struct InstallableCommand : virtual Args, SourceExprCommand
 
     std::optional<FlakeRef> getFlakeRefForCompletion() override
     {
-        return parseFlakeRef(_installable, absPath("."));
+        return parseFlakeRefWithFragment(_installable, absPath(".")).first;
     }
 
 private:

src/libcmd/installables.cc

@@ -905,10 +905,10 @@ std::optional<FlakeRef> InstallablesCommand::getFlakeRefForCompletion()
 {
     if (_installables.empty()) {
         if (useDefaultInstallables())
-            return parseFlakeRef(".", absPath("."));
+            return parseFlakeRefWithFragment(".", absPath(".")).first;
         return {};
     }
-    return parseFlakeRef(_installables.front(), absPath("."));
+    return parseFlakeRefWithFragment(_installables.front(), absPath(".")).first;
 }
 
 InstallableCommand::InstallableCommand()