Log debug statements around git operations, for easier debugging

[joeyAghion]

After last week's many branch renames, I checked on Horizon's cron and saw messages like this indicating that several projects' configurations were broken:

fatal: ambiguous argument '369fbf0245b91941b7685b69c937fb37d1b8243a..main/master': unknown revision or path not in the working tree.
Use '--' to separate paths from revisions, like this:
'git <command> [...] -- [...]'
fatal: ambiguous argument 'f588c0f4cfb410faf3e028a46f759c20fb3d3a36..main/master': unknown revision or path not in the working tree.
Use '--' to separate paths from revisions, like this:
'git <command> [...] -- [...]'
fatal: ambiguous argument 'ee99b953b67e9a4caadeaa39a03062e9d8312384..main/master': unknown revision or path not in the working tree.
Use '--' to separate paths from revisions, like this:
'git <command> [...] -- [...]'
fatal: ambiguous argument '04a9f7fd503267c99e8788a4733b189f6bf7113f..main/master': unknown revision or path not in the working tree.
Use '--' to separate paths from revisions, like this:
'git <command> [...] -- [...]'

However, I couldn't easily tell which projects were responsible for the errors. This PR adds debug-level log statements around each project refreshed by the recurring cron. By setting the LOG_LEVEL=0 environment variable, we should be able to more easily diagnose issues based on cron logs in the future. (I recommend unsetting it afterwards.)