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/checkpatch.py b/tools/patman/checkpatch.py
index e03cac1..2975881 100644
--- a/tools/patman/checkpatch.py
+++ b/tools/patman/checkpatch.py
@@ -8,8 +8,8 @@
 import re
 import sys
 
-from patman import gitutil
 from u_boot_pylib import command
+from u_boot_pylib import gitutil
 from u_boot_pylib import terminal
 
 EMACS_PREFIX = r'(?:[0-9]{4}.*\.patch:[0-9]+: )?'