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/t2203-add-intent.sh

@@ -232,17 +232,53 @@ test_expect_success 'double rename detection in status' '
 	)
 '
 
-test_expect_success 'diff-files/diff-cached shows ita as new/not-new files' '
+test_expect_success 'i-t-a files shown as new for "diff", "diff-files"; not-new for "diff --cached"' '
 	git reset --hard &&
-	echo new >new-ita &&
-	git add -N new-ita &&
-	git diff --summary >actual &&
-	echo " create mode 100644 new-ita" >expected &&
-	test_cmp expected actual &&
-	git diff --cached --summary >actual2 &&
-	test_must_be_empty actual2
-'
+	: >empty &&
+	content="foo" &&
+	echo "$content" >not-empty &&
 
+	hash_e=$(git hash-object empty) &&
+	hash_n=$(git hash-object not-empty) &&
+
+	cat >expect.diff_p <<-EOF &&
+	diff --git a/empty b/empty
+	new file mode 100644
+	index 0000000..$(git rev-parse --short $hash_e)
+	diff --git a/not-empty b/not-empty
+	new file mode 100644
+	index 0000000..$(git rev-parse --short $hash_n)
+	--- /dev/null
+	+++ b/not-empty
+	@@ -0,0 +1 @@
+	+$content
+	EOF
+	cat >expect.diff_s <<-EOF &&
+	 create mode 100644 empty
+	 create mode 100644 not-empty
+	EOF
+	cat >expect.diff_a <<-EOF &&
+	:000000 100644 0000000 0000000 A$(printf "\t")empty
+	:000000 100644 0000000 0000000 A$(printf "\t")not-empty
+	EOF
+
+	git add -N empty not-empty &&
+
+	git diff >actual &&
+	test_cmp expect.diff_p actual &&
+
+	git diff --summary >actual &&
+	test_cmp expect.diff_s actual &&
+
+	git diff-files -p >actual &&
+	test_cmp expect.diff_p actual &&
+
+	git diff-files --abbrev >actual &&
+	test_cmp expect.diff_a actual &&
+
+	git diff --cached >actual &&
+	test_must_be_empty actual
+'
 
 test_expect_success '"diff HEAD" includes ita as new files' '
 	git reset --hard &&