[git-bridge] Configurable `LOG_LEVEL` GitOrigin-RevId: 5dc34a3c980d9fd83898b4f96a30f60a8f9201ad
This allows our dependencies, such as jgit, to log at the debug level
We want all logs on stdout, for debugging, and warnings and errors on stderr, for remote monitoring. The SLF4J SimpleLogger can't seem to do this, but logback does, and we can still use the same SLF4J interface.