only ignore virtualenv directories at the top

To avoid collateral damage, only ignore the virtualenv directories if
they appear at the top of the working directory.

Also, only ignore lib64 if it is a directory (mostly to match the
other entries).

.gitignore

@@ -17,18 +17,18 @@ MANIFEST
 #   top of salt such as
 # - /some/path$ git clone https://github.com/thatch45/salt.git
 # - /some/path$ virtualenv --python=/usr/bin/python2.6 salt
-bin/
-etc/
-include/
-lib/
-lib64
-local/
-pip/
-share/
-tests/integration/tmp/
-tests/cachedir/
-tests/unit/templates/roots/
-var/
+/bin/
+/etc/
+/include/
+/lib/
+/lib64/
+/local/
+/pip/
+/share/
+/tests/integration/tmp/
+/tests/cachedir/
+/tests/unit/templates/roots/
+/var/
 
 # setuptools stuff
 *.egg-info