commit-graph: rename 'split_commit_graph_opts'

commit-graph: rename 'split_commit_graph_opts'

In the subsequent commit, additional options will be added to the
commit-graph API which have nothing to do with splitting.

Rename the 'split_commit_graph_opts' structure to the more-generic
'commit_graph_opts' to encompass both. Likewise, rename the 'flags'
member to instead be 'split_flags' to clarify that it only has to do
with the behavior implied by '--split'.

Suggested-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>