Make `nvm ls system` work.

nvm.sh

@@ -249,13 +249,20 @@ nvm_ls() {
         | sort -t. -u -k 1.2,1n -k 2,2n -k 3,3n | \grep -v '^ *\.' | \grep -e '^v'`
     fi
   fi
+
+  if nvm_has_system_node; then
+    if [ -z "$PATTERN" ]; then
+      VERSIONS="$VERSIONS$(printf '\n%s' 'system')"
+    elif [ "$PATTERN" = 'system' ]; then
+      VERSIONS="$(printf '%s' 'system')"
+    fi
+  fi
+
   if [ -z "$VERSIONS" ]; then
     echo "N/A"
     return 3
   fi
-  if [ -z "$PATTERN" ] && nvm_has_system_node; then
-    VERSIONS="$VERSIONS$(printf '\n%s' 'system')"
-  fi
+
   echo "$VERSIONS"
   return
 }

test/fast/Listing versions/Running "nvm ls system" should include "system" when appropriate

@@ -0,0 +1,22 @@
+#!/bin/sh
+
+cleanup () { unset -f nvm_has_system_node; }
+die () { cleanup ; echo $@ ; exit 1; }
+
+. ../../../nvm.sh
+
+mkdir -p ../../../v0.0.1
+mkdir -p ../../../v0.0.3
+mkdir -p ../../../v0.0.9
+mkdir -p ../../../v0.3.1
+mkdir -p ../../../v0.3.3
+mkdir -p ../../../v0.3.9
+
+nvm_has_system_node() { return 0; }
+nvm ls system | grep system 2>&1 > /dev/null
+[ $? -eq 0 ] || die '"nvm ls system" did not contain "system" when system node is present'
+
+nvm_has_system_node() { return 1; }
+nvm ls system | grep system 2>&1 > /dev/null
+[ $? -ne 0 ] || die '"nvm ls system" contained "system" when system node is not present'
+