Avoid rbenv-exec calling out to rbenv-version-name twice

Running any shim (and thus `rbenv-exec`) would always execute
`rbenv-version-name` twice: once in `rbenv-exec` and another time in
`rbenv-which`, even though RBENV_VERSION variable would have already
been populated at this point.

Now RBENV_VERSION is respected within `rbenv-which`.

libexec/rbenv-which

@@ -48,7 +48,6 @@ remove_from_path() {
   echo "${result%:}"
 }
 
-RBENV_VERSION="$(rbenv-version-name)"
 RBENV_COMMAND="$1"
 
 if [ -z "$RBENV_COMMAND" ]; then
@@ -56,6 +55,8 @@ if [ -z "$RBENV_COMMAND" ]; then
   exit 1
 fi
 
+RBENV_VERSION="${RBENV_VERSION:-$(rbenv-version-name)}"
+
 if [ "$RBENV_VERSION" = "system" ]; then
   PATH="$(remove_from_path "${RBENV_ROOT}/shims")"
   RBENV_COMMAND_PATH="$(command -v "$RBENV_COMMAND" || true)"
@@ -72,6 +73,9 @@ done
 
 if [ -x "$RBENV_COMMAND_PATH" ]; then
   echo "$RBENV_COMMAND_PATH"
+elif ! [ -d "${RBENV_ROOT}/versions/${RBENV_VERSION}" ]; then
+  echo "rbenv: version \`$RBENV_VERSION' is not installed" >&2
+  exit 1
 else
   echo "rbenv: $RBENV_COMMAND: command not found" >&2
 

test/which.bats

@@ -72,3 +72,12 @@ SH
   assert_success
   assert_output "HELLO=:hello:ugly:world:again"
 }
+
+@test "discovers version from rbenv-version-name" {
+  mkdir -p "$RBENV_ROOT"
+  cat > "${RBENV_ROOT}/version" <<<"1.8"
+  create_executable "1.8" "ruby"
+
+  RBENV_VERSION= run rbenv-which ruby
+  assert_success "${RBENV_ROOT}/versions/1.8/bin/ruby"
+}