Mark our source trees as safe for git to use as other users

git commit [1] introduced a new behaviour to work around a CVE that
disallows any git operations in directories not owned by the current
user.

This may seem unrelated to installation, but it plays havoc with PBR,
which calls out to git to get to get revision history.  So if you are
"pip install"-ing from a source tree you don't own, the PBR git calls
in that tree now fail and the install blows up.

This plays havoc with our model.  Firstly, we checkout all code as
"stack" then install it globally with "sudo" (i.e. root) -- which
breaks.  We also have cases of essentially the opposite -- checkouts
we have installed as root, but then run tox in them as a regular user;
tox wants to install the source in its venv but now we have another
user conflict.

This uses the only available configuration option to avoid that by
globally setting the source directories we clone as safe.  This is an
encroachment of the global system for sure, but is about the only
switch available at the moment.  For discussion of other approaches,
see [2].

Also, squashing the below backport which is needed for bionic
- https://review.opendev.org/q/I941ef5ea90970a0901236afe81c551aaf24ac1d8

Related-Bug: https://bugs.launchpad.net/devstack/+bug/1968798

[1] https://github.com/git/git/commit/8959555cee7ec045958f9b6dd62e541affb7e7d9
[2] https://review.opendev.org/c/openstack/devstack/+/837636

Change-Id: Ib9896a99b6d6c4d359ee412743ce30512b3c4fb7
(cherry picked from commit 676dcaf9)
(cherry picked from commit a9852abf)
(cherry picked from commit 16c57c51)
(cherry picked from commit c67e7d3b)
(cherry picked from commit 7f1025e9)
(cherry picked from commit 4da98ee6)