commit-graph: prefer default size_mult when given zero

In 50f26bd ("fetch: add fetch.writeCommitGraph config setting", 2019-09-02), the fetch builtin added the capability to write a commit-graph using the "--split" feature. This feature creates multiple commit-graph files, and those can merge based on a set of "split options" including a size multiple. The default size multiple is 2, which intends to provide a log_2 N depth of the commit-graph chain where N is the number of commits. However, I noticed during dogfooding that my commit-graph chains were becoming quite large when left only to builds by 'git fetch'. It turns out that in split_graph_merge_strategy(), we default the size_mult variable to 2 except we override it with the context's split_opts if they exist. In builtin/fetch.c, we create such a split_opts, but do not populate it with values. This problem is due to two failures: 1. It is unclear that we can add the flag COMMIT_GRAPH_WRITE_SPLIT with a NULL split_opts. 2. If we have a non-NULL split_opts, then we override the default values even if a zero value is given. Correct both of these issues. First, do not override size_mult when the options provide a zero value. Second, stop creating a split_opts in the fetch builtin. Signed-off-by: Derrick Stolee <dstolee@microsoft.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>