git_command: lru_cache needs maxsize for py36 & 37

Python 3.6 and 3.7 do not have a default value for lru_cache maxsize.
Not setting it would cause:
  TypeError: Expected maxsize to be an integer or None

Change-Id: I32d4fb6a0040a0c24da0b2f29f22f85a36c96531
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/388737
Reviewed-by: Mike Frysinger <vapier@google.com>
Tested-by: Daniel Kutik <daniel.kutik@lavawerk.com>
Commit-Queue: Daniel Kutik <daniel.kutik@lavawerk.com>
diff --git a/git_command.py b/git_command.py
index ef6e321..4b17f78 100644
--- a/git_command.py
+++ b/git_command.py
@@ -619,7 +619,7 @@
         self.git_stderr = git_stderr
 
     @property
-    @functools.lru_cache
+    @functools.lru_cache(maxsize=None)
     def suggestion(self):
         """Returns helpful next steps for the given stderr."""
         if not self.git_stderr: