Escape forward slashes in URI usernames/passwords properly

Summary: Fixes T11079. We currently un-escape / in phutil_escape_uri() because it is safe to leave unescaped in paths and an awful UX to escape, but it must be escaped in usernames/passwords.

Test Plan:

• Added failing unit test.
• Fixed test.
• Created a credential with username and password containing a slash.
• Set this credential on an observation URI for a repository.
• Used bin/repository update R... to pull the repostiory.
• Before patch, got an error about a malfored URI.
• After patch, things worked properly.

Cloning into bare repository '/Users/epriestley/dev/core/repo/local/55'...
fatal: unable to access 'https://slash/slash:slash/slash@github.com/epriestley/poems.git/': Could not resolve host: slash

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11079

Differential Revision: https://secure.phabricator.com/D16009