Summary: Sometimes restack attempts to clear preamend bookmarks on hidden commits. This only happens when the user has a configuration error (typically inhibit is disabled) which would mess up other parts of restack. Nonetheless, there is no reason to crash here.
Test Plan: Disable inhibit, perform an amend, and run `hg restack`. Unit test is forthcoming.
Reviewers: #mercurial, rmcelroy
Reviewed By: rmcelroy
Subscribers: rmcelroy, mjpieters
Differential Revision: https://phabricator.intern.facebook.com/D4856640
Tasks: 16726367
Signature: t1:4856640:1491816803:56beeb11360a7a2f6bc6243069c562b08a015afc
