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/settings.py b/tools/patman/settings.py
index 68c93e3..d66b22b 100644
--- a/tools/patman/settings.py
+++ b/tools/patman/settings.py
@@ -12,7 +12,7 @@
 import os
 import re
 
-from patman import gitutil
+from u_boot_pylib import gitutil
 
 """Default settings per-project.