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/patchstream.py b/tools/patman/patchstream.py
index 4955f6a..08795c4 100644
--- a/tools/patman/patchstream.py
+++ b/tools/patman/patchstream.py
@@ -15,9 +15,9 @@
 import tempfile
 
 from patman import commit
-from patman import gitutil
 from patman.series import Series
 from u_boot_pylib import command
+from u_boot_pylib import gitutil
 
 # Tags that we detect and remove
 RE_REMOVE = re.compile(r'^BUG=|^TEST=|^BRANCH=|^Review URL:'