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/test_checkpatch.py b/tools/patman/test_checkpatch.py
index db7860f..11d003b 100644
--- a/tools/patman/test_checkpatch.py
+++ b/tools/patman/test_checkpatch.py
@@ -11,10 +11,10 @@
 import unittest
 
 from patman import checkpatch
-from patman import gitutil
 from patman import patchstream
 from patman import series
 from patman import commit
+from u_boot_pylib import gitutil
 
 
 class Line: