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/t4027-diff-submodule.sh

@@ -36,7 +36,8 @@ test_expect_success setup '
 '
 
 test_expect_success 'git diff --raw HEAD' '
-	git diff --raw --abbrev=40 HEAD >actual &&
+	hexsz=$(test_oid hexsz) &&
+	git diff --raw --abbrev=$hexsz HEAD >actual &&
 	test_cmp expect actual
 '
 
@@ -245,23 +246,21 @@ test_expect_success 'git diff (empty submodule dir)' '
 '
 
 test_expect_success 'conflicted submodule setup' '
-
-	# 39 efs
-	c=fffffffffffffffffffffffffffffffffffffff &&
+	c=$(test_oid ff_1) &&
 	(
 		echo "000000 $ZERO_OID 0	sub" &&
 		echo "160000 1$c 1	sub" &&
 		echo "160000 2$c 2	sub" &&
 		echo "160000 3$c 3	sub"
 	) | git update-index --index-info &&
-	echo >expect.nosub '\''diff --cc sub
+	echo >expect.nosub "diff --cc sub
 index 2ffffff,3ffffff..0000000
 --- a/sub
 +++ b/sub
 @@@ -1,1 -1,1 +1,1 @@@
-- Subproject commit 2fffffffffffffffffffffffffffffffffffffff
- -Subproject commit 3fffffffffffffffffffffffffffffffffffffff
-++Subproject commit 0000000000000000000000000000000000000000'\'' &&
+- Subproject commit 2$c
+ -Subproject commit 3$c
+++Subproject commit $ZERO_OID" &&
 
 	hh=$(git rev-parse HEAD) &&
 	sed -e "s/$ZERO_OID/$hh/" expect.nosub >expect.withsub