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/t4064-diff-oidfind.sh

@@ -65,4 +65,59 @@ test_expect_success 'find a submodule' '
 	test_cmp expect actual
 '
 
+test_expect_success 'set up merge tests' '
+	test_commit base &&
+
+	git checkout -b boring base^ &&
+	echo boring >file &&
+	git add file &&
+	git commit -m boring &&
+
+	git checkout -b interesting base^ &&
+	echo interesting >file &&
+	git add file &&
+	git commit -m interesting &&
+
+	blob=$(git rev-parse interesting:file)
+'
+
+test_expect_success 'detect merge which introduces blob' '
+	git checkout -B merge base &&
+	git merge --no-commit boring &&
+	echo interesting >file &&
+	git commit -am "introduce blob" &&
+	git diff-tree --format=%s --find-object=$blob -c --name-status HEAD >actual &&
+	cat >expect <<-\EOF &&
+	introduce blob
+
+	AM	file
+	EOF
+	test_cmp expect actual
+'
+
+test_expect_success 'detect merge which removes blob' '
+	git checkout -B merge interesting &&
+	git merge --no-commit base &&
+	echo boring >file &&
+	git commit -am "remove blob" &&
+	git diff-tree --format=%s --find-object=$blob -c --name-status HEAD >actual &&
+	cat >expect <<-\EOF &&
+	remove blob
+
+	MA	file
+	EOF
+	test_cmp expect actual
+'
+
+test_expect_success 'do not detect merge that does not touch blob' '
+	git checkout -B merge interesting &&
+	git merge -m "untouched blob" base &&
+	git diff-tree --format=%s --find-object=$blob -c --name-status HEAD >actual &&
+	cat >expect <<-\EOF &&
+	untouched blob
+
+	EOF
+	test_cmp expect actual
+'
+
 test_done