merge(third_party/git): Merge squashed git subtree at v2.23.0

Merge commit '1b593e1ea4' as 'third_party/git'

third_party/git/ci/run-test-slice.sh

@@ -0,0 +1,17 @@
+#!/bin/sh
+#
+# Test Git in parallel
+#
+
+. ${0%/*}/lib.sh
+
+case "$CI_OS_NAME" in
+windows*) cmd //c mklink //j t\\.prove "$(cygpath -aw "$cache_dir/.prove")";;
+*) ln -s "$cache_dir/.prove" t/.prove;;
+esac
+
+make --quiet -C t T="$(cd t &&
+	./helper/test-tool path-utils slice-tests "$1" "$2" t[0-9]*.sh |
+	tr '\n' ' ')"
+
+check_unignored_build_artifacts