From: John Cai <johncai86@gmail.com>

44451a2e5e (attr: teach "--attr-source=<tree>" global option to "git",
2023-05-06) provided the ability to pass in a treeish as the attr
source. When a revision does not resolve to a valid tree is passed, Git
will die. GitLab keeps bare repositories and always reads attributes
from the default branch, so we pass in HEAD to --attr-source.

With empty repositories however, HEAD does not point to a valid treeish,
causing Git to die. This means we would need to check for a valid
treeish each time. To avoid this, let's add a configuration that allows
Git to simply ignore --attr-source if it does not resolve to a valid
tree.

Signed-off-by: John Cai <johncai86@gmail.com>

Submitted-As: https://lore.kernel.org/git/pull.1577.git.git.1695218431033.gitgitgadget@gmail.com