commit-graph: use progress title directly

commit-graph: use progress title directly

merge_commit_graphs() copies the (translated) progress message into a
strbuf and passes the copy to start_delayed_progress() at each loop
iteration.  The latter function takes a string pointer, so let's avoid
the detour and hand the string to it directly.  That's shorter, simpler
and slightly more efficient.

Signed-off-by: René Scharfe <l.s.r@web.de>
Acked-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>