Simplify parsing branch switching events in reflog

We only accept "checkout: moving from A to B" newer style reflog entries,
in order to pick up A.  There is no point computing where B begins at
after running strstr to locate " to ", nor adding 4 and then subtracting 4
from the same pointer.

Signed-off-by: Junio C Hamano <gitster@pobox.com>