Use correct syntax for git diff (#915)

Author: ktf

alibuild_helpers/git.py

@@ -74,7 +74,7 @@ def setWriteUrlCmd(self, url):
     return ["remote", "set-url", "--push", "origin", url]
 
   def diffCmd(self, directory):
-    return "cd %s && git diff -r HEAD && git status --porcelain" % directory
+    return "cd %s && git diff HEAD && git status --porcelain" % directory
 
   def checkUntracked(self, line):
     return line.startswith("?? ")