Only copy a commit if it has at least one nonidentical parent.

Only copy a commit if it has at least one nonidentical parent.

This is a simplification of the previous logic.  I don't *think* it'll break
anything.

Results in far fewer useless merge commmits when playing with gitweb in the
git project:

git subtree split --prefix=gitweb --annotate='(split) ' 0a8f4f0^^..f2e7330
	--onto=1130ef3

...and it doesn't *seem* to eliminate anything important.