This splits out a common routine that parses a single line of
attribute file and adds it to the attr_stack.  It should not
change any behaviour, other than attrs array in the attr_stack
structure is now grown with alloc_nr() macro, instead of one by
one, which relied on xrealloc() to give enough slack to be
efficient enough.

Signed-off-by: Junio C Hamano <gitster@pobox.com>