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/t5000-tar-tree.sh

@@ -94,6 +94,16 @@ check_tar() {
 	'
 }
 
+check_added() {
+	dir=$1
+	path_in_fs=$2
+	path_in_archive=$3
+
+	test_expect_success " validate extra file $path_in_archive" '
+		diff -r $path_in_fs $dir/$path_in_archive
+	'
+}
+
 test_expect_success 'setup' '
 	test_oid_cache <<-EOF
 	obj sha1:19f9c8273ec45a8938e6999cb59b3ff66739902a
@@ -164,6 +174,25 @@ test_expect_success 'git-archive --prefix=olde-' '
 
 check_tar with_olde-prefix olde-
 
+test_expect_success 'git archive --add-file' '
+	echo untracked >untracked &&
+	git archive --add-file=untracked HEAD >with_untracked.tar
+'
+
+check_tar with_untracked
+check_added with_untracked untracked untracked
+
+test_expect_success 'git archive --add-file twice' '
+	echo untracked >untracked &&
+	git archive --prefix=one/ --add-file=untracked \
+		--prefix=two/ --add-file=untracked \
+		--prefix= HEAD >with_untracked2.tar
+'
+
+check_tar with_untracked2
+check_added with_untracked2 untracked one/untracked
+check_added with_untracked2 untracked two/untracked
+
 test_expect_success 'git archive on large files' '
     test_config core.bigfilethreshold 1 &&
     git archive HEAD >b3.tar &&