patman: Fix detection of git version

A missing 'global' declaration means that this feature does not currently
work. Fix it.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/patman/gitutil.py b/tools/patman/gitutil.py
index fbd170f..80edc7c 100644
--- a/tools/patman/gitutil.py
+++ b/tools/patman/gitutil.py
@@ -481,6 +481,8 @@
 def Setup():
     """Set up git utils, by reading the alias files."""
     # Check for a git alias file also
+    global use_no_decorate
+
     alias_fname = GetAliasFile()
     if alias_fname:
         settings.ReadGitAliases(alias_fname)