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/func_test.py b/tools/patman/func_test.py
index af6c025..bf333dc 100644
--- a/tools/patman/func_test.py
+++ b/tools/patman/func_test.py
@@ -18,11 +18,11 @@
 
 from patman.commit import Commit
 from patman import control
-from patman import gitutil
 from patman import patchstream
 from patman.patchstream import PatchStream
 from patman.series import Series
 from patman import settings
+from u_boot_pylib import gitutil
 from u_boot_pylib import terminal
 from u_boot_pylib import tools
 from u_boot_pylib.test_util import capture_sys_output