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/t3700-add.sh

@@ -192,7 +192,7 @@ test_expect_success 'git add --refresh with pathspec' '
 	test_must_be_empty actual &&
 
 	git diff-files --name-only >actual &&
-	! grep bar actual&&
+	! grep bar actual &&
 	grep baz actual
 '
 
@@ -326,7 +326,9 @@ test_expect_success 'git add --dry-run of an existing file output' "
 cat >expect.err <<\EOF
 The following paths are ignored by one of your .gitignore files:
 ignored-file
-Use -f if you really want to add them.
+hint: Use -f if you really want to add them.
+hint: Turn this message off by running
+hint: "git config advice.addIgnoredFile false"
 EOF
 cat >expect.out <<\EOF
 add 'track-this'