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/lib-credential.sh

@@ -1,4 +1,5 @@
-#!/bin/sh
+# Shell library for testing credential handling including helpers. See t0302
+# for an example of testing a specific helper.
 
 # Try a set of credential helpers; the expected stdin,
 # stdout and stderr should be provided on stdin,
@@ -19,7 +20,7 @@ check() {
 		false
 	fi &&
 	test_cmp expect-stdout stdout &&
-	test_cmp expect-stderr stderr
+	test_i18ncmp expect-stderr stderr
 }
 
 read_chunk() {