There are too many repetitious "I have this new attr_stack element;
push it at the top of the stack" sequence. The new helper function
push_stack() gives us a way to express what is going on at these
places, and as a side effect, halves the number of times we mention
the attr_stack global variable.
Signed-off-by: Junio C Hamano <gitster@pobox.com> Signed-off-by: Stefan Beller <sbeller@google.com> Signed-off-by: Brandon Williams <bmwill@google.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>