ignore non-existent refs in dwim_log()

ignore non-existent refs in dwim_log()

f2eba66 (Enable HEAD@{...} and make it independent from the current
branch, 2007-02-03) introduced dwim_log() to handle <refname>@{...}
syntax, and as part of its processing, it checks if the ref exists by
calling refsolve_ref().  It should call it as a reader to make sure the
call returns NULL for a nonexistent ref (not as a potential writer in
which case it does not return NULL).

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