R1:83edbffd521d

Config page: add lovely git-related error messages in standard error log

Summary:
Premise: the Config page runs git commands. Spoiler: they can fail.

Before this change errors were just suppressed and ignored.

After this change you get at least a log line. Also, you get a tip for a very specific well-known error affecting recent git.

Probably suppressing stuff was fine in the moment git worked here. But nowadays
git doesn't work so easily here, since it introduced very weird additional
configurations…