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/project.py b/project.py
index b975b72..9d7476b 100644
--- a/project.py
+++ b/project.py
@@ -59,7 +59,7 @@
 # +-10% random jitter is added to each Fetches retry sleep duration.
 RETRY_JITTER_PERCENT = 0.1
 
-# Whether to use alternates.
+# Whether to use alternates.  Switching back and forth is *NOT* supported.
 # TODO(vapier): Remove knob once behavior is verified.
 _ALTERNATES = os.environ.get('REPO_USE_ALTERNATES') == '1'