docs: document security issues around untrusted .git dirs

docs: document security issues around untrusted .git dirs

For a long time our general philosophy has been that it's unsafe to run
arbitrary Git commands if you don't trust the hooks or config in .git,
but that running upload-pack should be OK. E.g., see 1456b043fc (Remove
post-upload-hook, 2009-12-10), or the design of uploadpack.packObjectsHook.

But we never really documented this (and even the discussions that led
to 1456b043fc were not on the public list!). Let's try to make our
approach more clear, but also be realistic that even upload-pack carries
some risk.

Helped-by: Filip Hejsek <filip.hejsek@gmail.com>
Helped-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>