git-merge: minor fix for no_trivial_merge_strategies.

git-merge: minor fix for no_trivial_merge_strategies.

The shell loop to determine if we should skip the trivial
in-index merge stage based on what strategy is given was not
prepared to have more than one strategy listed in the variable
$no_trivial_merge_strategies.

This does not trigger unless you use a modified git but the fix
is simple and straightforward, so let's fix it before 1.5.0.1.

Signed-off-by: Junio C Hamano <junkio@cox.net>