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.

git-subtree.sh

@@ -258,6 +258,7 @@ copy_or_skip()
 	assert [ -n "$tree" ]
 
 	identical=
+	nonidentical=
 	p=
 	gotparents=
 	for parent in $newparents; do
@@ -266,6 +267,8 @@ copy_or_skip()
 		if [ "$ptree" = "$tree" ]; then
 			# an identical parent could be used in place of this rev.
 			identical="$parent"
+		else
+			nonidentical="$parent"
 		fi
 		
 		# sometimes both old parents map to the same newparent;
@@ -283,7 +286,7 @@ copy_or_skip()
 		fi
 	done
 	
-	if [ -n "$identical" -a "$gotparents" = " $identical" ]; then
+	if [ -n "$identical" -a -z "$nonidentical" ]; then
 		echo $identical
 	else
 		copy_commit $rev $tree "$p" || exit $?