mq: strip all leading slashes from url when importing
When trying to import a url that ends with a slash
os.path.basename() would return an empty string. So
when seeing a leading slash, remove it and infer the
patch name from the remaining url.
e.g. `hg qimport http://paste.pocoo.org/raw/xxx/`
deduced '.' to be the patch name. Now we'll deduce 'xxx'.