cherry-pick: do not error on non-merge commits when '-m 1' is specified

cherry-pick: do not error on non-merge commits when '-m 1' is specified

When cherry-picking multiple commits, it's impossible to have both
merge- and non-merge commits on the same command-line. Not specifying
'-m 1' results in cherry-pick refusing to handle merge commits, while
specifying '-m 1' fails on non-merge commits.

This patch allows '-m 1' for non-merge commits. As mainline is always
the only parent for a non-merge commit, it makes little sense to
disable it.

Signed-off-by: Sergey Organov <sorganov@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>