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/func_test.py b/tools/buildman/func_test.py
index 4e12c67..c7c4f50 100644
--- a/tools/buildman/func_test.py
+++ b/tools/buildman/func_test.py
@@ -18,8 +18,8 @@
 from buildman import cmdline
 from buildman import control
 from buildman import toolchain
-from patman import gitutil
 from u_boot_pylib import command
+from u_boot_pylib import gitutil
 from u_boot_pylib import terminal
 from u_boot_pylib import test_util
 from u_boot_pylib import tools