sync: clear preciousObjects when set in error.

If this is a project that is not using object sharing (there is only one
copy of the remote project) then clear preciousObjects.

To override this for a project, run:

  git config --replace-all repo.preservePreciousObjects true

Change-Id: If3ea061c631c5ecd44ead84f68576012e2c7405c
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/350235
Reviewed-by: Jonathan Nieder <jrn@google.com>
Tested-by: LaMont Jones <lamontjones@google.com>
diff --git a/git_config.py b/git_config.py
index 98cade3..94378e9 100644
--- a/git_config.py
+++ b/git_config.py
@@ -219,8 +219,8 @@
     """Set the value(s) for a key.
        Only this configuration file is modified.
 
-       The supplied value should be either a string,
-       or a list of strings (to store multiple values).
+       The supplied value should be either a string, or a list of strings (to
+       store multiple values), or None (to delete the key).
     """
     key = _key(name)