commit-graph: avoid malloc'ing a local variable

commit-graph: avoid malloc'ing a local variable

We do need a context to write the commit graph, but that context is only
needed during the life time of `commit_graph_write()`, therefore it can
easily be a stack variable.

This also helps CodeQL recognize that it is safe to assign the address
of other local variables to the context's fields.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>