merge: let custom strategies intervene in trivial merges

merge: let custom strategies intervene in trivial merges

As v1.6.1-rc1~294^2 (2008-08-23) explains, custom merge strategies
do not even kick in when the merge is truly trivial.  But they
should, since otherwise a custom “--strategy=theirs” is not useful.

Perhaps custom strategies should not allow fast-forward either.  This
patch does not make that change, since it is less important (because
it is always possible to explicitly use --no-ff).

Reported-by: Yaroslav Halchenko <debian@onerussian.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>