patman: Fix detection of git version

A missing 'global' declaration means that this feature does not currently
work. Fix it.
Signed-off-by: NSimon Glass <sjg@chromium.org>

tools/patman/gitutil.py

@@ -481,6 +481,8 @@ def GetDefaultUserEmail():
 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)