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/t5801-remote-helpers.sh

@@ -11,9 +11,15 @@ test_description='Test remote-helper import and export commands'
 PATH="$TEST_DIRECTORY/t5801:$PATH"
 
 compare_refs() {
+	fail= &&
+	if test "x$1" = 'x!'
+	then
+		fail='!' &&
+		shift
+	fi &&
 	git --git-dir="$1/.git" rev-parse --verify $2 >expect &&
 	git --git-dir="$3/.git" rev-parse --verify $4 >actual &&
-	test_cmp expect actual
+	eval $fail test_cmp expect actual
 }
 
 test_expect_success 'setup repository' '
@@ -189,7 +195,7 @@ test_expect_success GPG 'push signed tag' '
 	git push origin signed-tag
 	) &&
 	compare_refs local signed-tag^{} server signed-tag^{} &&
-	test_must_fail compare_refs local signed-tag server signed-tag
+	compare_refs ! local signed-tag server signed-tag
 '
 
 test_expect_success GPG 'push signed tag with signed-tags capability' '
@@ -247,7 +253,6 @@ clean_mark () {
 test_expect_success 'proper failure checks for fetching' '
 	(cd local &&
 	test_must_fail env GIT_REMOTE_TESTGIT_FAILURE=1 git fetch 2>error &&
-	cat error &&
 	test_i18ngrep -q "error while running fast-import" error
 	)
 '