dumb: Fix regression when joining URLs

urljoin does not produce the same output if the base URL does not
have a trailing slash.

from urllib.parse import urljoin
urljoin("https://git.example.org/repo", "info/refs")

'https://git.example.org/info/refs'

urljoin("https://git.example.org/repo/", "info/refs")

'https://git.example.org/repo/info/refs'

So ensure the base URL ends with a slash to avoid generating invalid
URLs and make loading failed.