Git SSH Urls cannot select a branch

[cmellard]

When using envbuilder with a git ssh url we cannot target a specific branch. Tracing this through it's because of the giturls library succeeds in parsing a transport/http regex and gleans the url fragment and this is mapped to a branch in coder. However for ssh no fragment is parsed in the scpProvider regex. Presumably it would be good to have a way similar to https to signal to coder a branch we want to check out. If this can't be done via the Git url then perhaps another env var / parameter needs to be available to achieve this