diff-merges: move checks for first_parent_only out of the module

diff-merges: move checks for first_parent_only out of the module

The checks for first_parent_only don't in fact belong to this module,
as the primary purpose of this flag is history traversal limiting, so
get it out of this module and rename the

diff_merges_first_parent_defaults_to_enable()

to

diff_merges_default_to_enable()

to match new semantics.

Signed-off-by: Sergey Organov <sorganov@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>