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/t5545-push-options.sh

@@ -115,7 +115,7 @@ test_expect_success 'push options and submodules' '
 
 	git -C parent submodule add ../upstream workbench &&
 	git -C parent/workbench remote add up ../../upstream &&
-	git -C parent commit -m "add submoule" &&
+	git -C parent commit -m "add submodule" &&
 
 	test_commit -C parent/workbench two &&
 	git -C parent add workbench &&
@@ -278,4 +278,7 @@ test_expect_success 'push options keep quoted characters intact (http)' '
 	test_cmp expect "$HTTPD_DOCUMENT_ROOT_PATH"/upstream.git/hooks/pre-receive.push_options
 '
 
+# DO NOT add non-httpd-specific tests here, because the last part of this
+# test script is only executed when httpd is available and enabled.
+
 test_done