[Perf] nvm_resolve_local_alias: skip nvm_version for an exact, installed version

See #3416

nvm.sh

@@ -1673,11 +1673,18 @@ nvm_resolve_local_alias() {
   if [ -z "${VERSION}" ]; then
     return $EXIT_CODE
   fi
-  if [ "_${VERSION}" != '_∞' ]; then
-    nvm_version "${VERSION}"
-  else
+  if [ "_${VERSION}" = '_∞' ]; then
     nvm_echo "${VERSION}"
+    return
   fi
+
+  # Fast path: an exact, installed vX.Y.Z needs no further resolution.
+  if nvm_is_installed_exact_version "${VERSION}"; then
+    nvm_echo "${VERSION}"
+    return
+  fi
+
+  nvm_version "${VERSION}"
 }
 
 nvm_iojs_prefix() {

test/fast/Aliases/nvm_resolve_local_alias skips nvm_version for an exact installed version

@@ -0,0 +1,20 @@
+#!/bin/sh
+
+die () { echo "$@" ; exit 1; }
+
+: nvm.sh
+\. ../../../nvm.sh
+
+nvm_version() {
+  echo 'nvm_version was called' >&2
+  return 42
+}
+
+EXACT="$(nvm_resolve_local_alias test-stable-1)"
+[ "_$EXACT" = "_v0.0.1" ] || die "'nvm_resolve_local_alias test-stable-1' was not v0.0.1; got $EXACT"
+
+echo v0.0.99 > ../../../alias/test-not-installed
+NOT_INSTALLED="$(nvm_resolve_local_alias test-not-installed 2>/dev/null)"
+EXIT_CODE=$?
+rm -f ../../../alias/test-not-installed
+[ "_$EXIT_CODE" = "_42" ] || die "an exact version that is not installed should still go through nvm_version; got exit code $EXIT_CODE"