cmd_pull didn't support --squash correctly.

We should implement it as
	git fetch ...
	git subtree merge ...

But we were instead just calling
	git pull -s subtree ...

because 'git subtree merge' used to be just an alias for 'git merge -s
subtree', but it no longer is.

git-subtree.sh

@@ -567,8 +567,9 @@ cmd_merge()
 cmd_pull()
 {
 	ensure_clean
-	set -x
-	git pull -s subtree "$@"
+	git fetch "$@" || exit $?
+	revs=FETCH_HEAD
+	cmd_merge
 }
 
 "cmd_$command" "$@"