treeproof (and consequently monitor_submissions) crash on invalid `git_repository_url`

[mentonin]

Description

_define_tree_name on treeproof.py uses a regex to guess tree names from URL, which crashes if git_repository_branch is malformed (ends with a slash or does not have a slash) and may silently mislabel urls if they don't end in <tree-name>/<subtree-name>[.extension]

Related issues:

• RFC: Analysis-oriented database schema #1947