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/t5514-fetch-multiple.sh

@@ -108,7 +108,7 @@ test_expect_success 'git fetch --multiple (two remotes)' '
 	 GIT_TRACE=1 git fetch --multiple one two 2>trace &&
 	 git branch -r > output &&
 	 test_cmp ../expect output &&
-	 grep "built-in: git gc" trace >gc &&
+	 grep "built-in: git maintenance" trace >gc &&
 	 test_line_count = 1 gc
 	)
 '
@@ -183,4 +183,15 @@ test_expect_success 'git fetch --all --tags' '
 	test_cmp expect test8/output
 '
 
+test_expect_success 'parallel' '
+	git remote add one ./bogus1 &&
+	git remote add two ./bogus2 &&
+
+	test_must_fail env GIT_TRACE="$PWD/trace" \
+		git fetch --jobs=2 --multiple one two 2>err &&
+	grep "preparing to run up to 2 tasks" trace &&
+	test_i18ngrep "could not fetch .one.*128" err &&
+	test_i18ngrep "could not fetch .two.*128" err
+'
+
 test_done