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/buildman/builderthread.py b/tools/buildman/builderthread.py
index 29e6cf3..78c95a6 100644
--- a/tools/buildman/builderthread.py
+++ b/tools/buildman/builderthread.py
@@ -17,8 +17,8 @@
 import threading
 
 from buildman import cfgutil
-from patman import gitutil
 from u_boot_pylib import command
+from u_boot_pylib import gitutil
 from u_boot_pylib import tools
 
 RETURN_CODE_RETRY = -1