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/t0061-run-command.sh

@@ -12,6 +12,10 @@ cat >hello-script <<-EOF
 	cat hello-script
 EOF
 
+test_expect_success MINGW 'subprocess inherits only std handles' '
+	test-tool run-command inherited-handle
+'
+
 test_expect_success 'start_command reports ENOENT (slash)' '
 	test-tool run-command start-command-ENOENT ./does-not-exist 2>err &&
 	test_i18ngrep "\./does-not-exist" err
@@ -210,10 +214,23 @@ test_expect_success MINGW 'verify curlies are quoted properly' '
 	test_cmp expect actual
 '
 
-test_expect_success MINGW 'can spawn with argv[0] containing spaces' '
-	cp "$GIT_BUILD_DIR/t/helper/test-fake-ssh$X" ./ &&
-	test_must_fail "$PWD/test-fake-ssh$X" 2>err &&
-	grep TRASH_DIRECTORY err
+test_expect_success MINGW 'can spawn .bat with argv[0] containing spaces' '
+	bat="$TRASH_DIRECTORY/bat with spaces in name.bat" &&
+
+	# Every .bat invocation will log its arguments to file "out"
+	rm -f out &&
+	echo "echo %* >>out" >"$bat" &&
+
+	# Ask git to invoke .bat; clone will fail due to fake SSH helper
+	test_must_fail env GIT_SSH="$bat" git clone myhost:src ssh-clone &&
+
+	# Spawning .bat can fail if there are two quoted cmd.exe arguments.
+	# .bat itself is first (due to spaces in name), so just one more is
+	# needed to verify. GIT_SSH will invoke .bat multiple times:
+	# 1) -G myhost
+	# 2) myhost "git-upload-pack src"
+	# First invocation will always succeed. Test the second one.
+	grep "git-upload-pack" out
 '
 
 test_done