u_boot_pylib: Move gitutil into the library

Move this file into U-Boot's Python library, so that it is no-longer
part of patman.

This makes a start on:

https://source.denx.de/u-boot/custodians/u-boot-dm/-/issues/35

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/patman/get_maintainer.py b/tools/patman/get_maintainer.py
index 8df3d12..200ee96 100644
--- a/tools/patman/get_maintainer.py
+++ b/tools/patman/get_maintainer.py
@@ -7,8 +7,8 @@
 import shlex
 import shutil
 
-from patman import gitutil
 from u_boot_pylib import command
+from u_boot_pylib import gitutil
 
 
 def find_get_maintainer(script_file_name):