commit-graph: remove a duplicate assignment

commit-graph: remove a duplicate assignment

Leave the variable 'g' uninitialized before it is set just before its
first use in front of a loop, which is a much more appropriate place to
indicate what it is used for.

Also initialize the variable 'num_commits' just before the loop instead
of at the beginning of the function for the same reason.

Reviewed-by: Derrick Stolee <stolee@gmail.com>
Signed-off-by: Alex Henrie <alexhenrie24@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>