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/control.py b/tools/buildman/control.py
index 55d4d77..5109b1c 100644
--- a/tools/buildman/control.py
+++ b/tools/buildman/control.py
@@ -20,9 +20,9 @@
 from buildman import cfgutil
 from buildman import toolchain
 from buildman.builder import Builder
-from patman import gitutil
 from patman import patchstream
 from u_boot_pylib import command
+from u_boot_pylib import gitutil
 from u_boot_pylib import terminal
 from u_boot_pylib import tools
 from u_boot_pylib.terminal import print_clear, tprint