commit-reach: fix cast in compare_commits_by_gen()

commit-reach: fix cast in compare_commits_by_gen()

The elements of the array to be sorted are commit pointers, so the
comparison function gets handed references to these pointers, not
pointers to commit objects.  Cast to the right type and dereference
once to correctly get the commit reference.

Found using Clang's ASan and t5500.

Signed-off-by: Rene Scharfe <redacted>
Signed-off-by: Junio C Hamano <redacted>