setup: opt-out of check with safe.directory=*

With the addition of the safe.directory in 8959555ce
(setup_git_directory(): add an owner check for the top-level directory,
2022-03-02) released in v2.35.2, we are receiving feedback from a
variety of users about the feature.

Some users have a very large list of shared repositories and find it
cumbersome to add this config for every one of them.

In a more difficult case, certain workflows involve running Git commands
within containers. The container boundary prevents any global or system
config from communicating `safe.directory` values from the host into the
container. Further, the container almost always runs as a different user
than the owner of the directory in the host.

To simplify the reactions necessary for these users, extend the
definition of the safe.directory config value to include a possible '*'
value. This value implies that all directories are safe, providing a
single setting to opt-out of this protection.

Note that an empty assignment of safe.directory clears all previous
values, and this is already the case with the "if (!value || !*value)"
condition.

Signed-off-by: Derrick Stolee <derrickstolee@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t0033-safe-directory.sh

@@ -36,4 +36,14 @@ test_expect_success 'safe.directory matches, but is reset' '
 	expect_rejected_dir
 '
 
+test_expect_success 'safe.directory=*' '
+	git config --global --add safe.directory "*" &&
+	git status
+'
+
+test_expect_success 'safe.directory=*, but is reset' '
+	git config --global --add safe.directory "" &&
+	expect_rejected_dir
+'
+
 test_done