git: Don't use an empty pathspec when listing

An empty string as a pathspec element matches all paths, but git has
recently started complaining about it, as it could be problematic for
some operations like rm. In the future, it will be considered an error.

So this patch uses "." instead of the empty pathspec, as recommended.

d426430e6e

git.py

@@ -525,7 +525,10 @@ class Tree:
             cmd.t = None
 
         cmd.arg(self.ref)
-        cmd.arg(path)
+        if not path:
+            cmd.arg(".")
+        else:
+            cmd.arg(path)
 
         for l in cmd.run():
             _mode, otype, _oid, size, name = l.split(None, 4)