merge-recursive: use common name for ancestors/common/base_list

merge-recursive: use common name for ancestors/common/base_list

merge_trees(), merge_recursive(), and merge_recursive_generic() in
their function headers used four different names for the merge base or
list of merge bases they were passed:
  * 'common'
  * 'ancestors'
  * 'ca'
  * 'base_list'
They were able to refer to it four different ways instead of only three
by using a different name in the signature for the .c file than the .h
file.  Change all of these to 'merge_base' or 'merge_bases'.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>