Improve checking for existence of the --prefix directory.

For add, the prefix must *not* already exist.  For all the other commands,
it *must* already exist.

git-subtree.sh

@@ -106,9 +106,12 @@ if [ -z "$prefix" ]; then
 	die "You must provide the --prefix option."
 fi
 
-if [ "$command" = "split" -a """"! -e "$prefix" ]; then
-	die "$prefix does not exist."
-fi
+case "$command" in
+	add) [ -e "$prefix" ] && 
+		die "prefix '$prefix' already exists." ;;
+	*)   [ -e "$prefix" ] || 
+		die "'$prefix' does not exist; use 'git subtree add'" ;;
+esac
 
 dir="$(dirname "$prefix/.")"
 

test.sh

@@ -21,6 +21,19 @@ check()
 	fi
 }
 
+check_not()
+{
+	echo
+	echo "check: NOT " "$@"
+	if "$@"; then
+		echo FAILED
+		exit 1
+	else
+		echo ok
+		return 0
+	fi
+}
+
 check_equal()
 {
 	echo
@@ -94,6 +107,8 @@ git fetch ../subproj sub1
 git branch sub1 FETCH_HEAD
 
 # check if --message works for add
+check_not git subtree merge --prefix=subdir sub1
+check_not git subtree pull --prefix=subdir ../subproj sub1
 git subtree add --prefix=subdir --message="Added subproject" sub1
 check_equal "$(last_commit_message)" "Added subproject"
 undo