merge(3p/git): Merge git subtree at v2.29.2

This also bumps the stable nixpkgs to 20.09 as of 2020-11-21, because
there is some breakage in the git build related to the netrc
credentials helper which someone has taken care of in nixpkgs.

The stable channel is not used for anything other than git, so this
should be fine.

Change-Id: I3575a19dab09e1e9556cf8231d717de9890484fb

third_party/git/t/helper/test-advise.c

@@ -0,0 +1,22 @@
+#include "test-tool.h"
+#include "cache.h"
+#include "advice.h"
+#include "config.h"
+
+int cmd__advise_if_enabled(int argc, const char **argv)
+{
+	if (argc != 2)
+		die("usage: %s <advice>", argv[0]);
+
+	setup_git_directory();
+	git_config(git_default_config, NULL);
+
+	/*
+	 * Any advice type can be used for testing, but NESTED_TAG was
+	 * selected here and in t0018 where this command is being
+	 * executed.
+	 */
+	advise_if_enabled(ADVICE_NESTED_TAG, argv[1]);
+
+	return 0;
+}