style(3p/nix): Enforce braces around loops and conditionals

This change was generated with:

  fd -e cc -e hh | xargs -I{} clang-tidy {} -p ~/projects/nix-build/ \
    --checks='-*,readability-braces-around-statements' --fix \
    -fix-errors

Some manual fixes were applied because some convoluted unbraced
statements couldn't be untangled by clang-tidy.

This commit still includes invalid files, but I decided to clean them
up in a subsequent commit so that it becomes more obvious where
clang-tidy failed. Maybe this will allow for a bug-report to
clang-tidy.

third_party/nix/src/libexpr/primops/fetchMercurial.cc

@@ -203,8 +203,10 @@ static void prim_fetchMercurial(EvalState& state, const Pos& pos, Value** args,
     if (url.empty())
       throw EvalError(format("'url' argument required, at %1%") % pos);
 
-  } else
-    url = state.coerceToString(pos, *args[0], context, false, false);
+  } else {
+    url
+  }
+  = state.coerceToString(pos, *args[0], context, false, false);
 
   // FIXME: git externals probably can be used to bypass the URI
   // whitelist. Ah well.