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.
Test Plan
Tests have been updated to cover that case.
Migrated from D6917 (view on Phabricator)