Fix workon_cwd to detect git subtree

In a `git subtree` working directory, `.git` is a file containing a path
to the parent git working direcotry. This change attempts to find a
virtualenv for a subtree.

plugins/virtualenvwrapper/virtualenvwrapper.plugin.zsh

@@ -38,7 +38,7 @@ if [[ ! $DISABLE_VENV_CD -eq 1 ]]; then
       # Get absolute path, resolving symlinks
       local PROJECT_ROOT="${PWD:A}"
       while [[ "$PROJECT_ROOT" != "/" && ! -e "$PROJECT_ROOT/.venv" \
-          && ! -d "$PROJECT_ROOT/.git" ]]; do
+          && ! -e "$PROJECT_ROOT/.git" ]]; do
         PROJECT_ROOT="${PROJECT_ROOT:h}"
       done