rebase: catch RepoLookupError at restoring rebase state for summary Before this patch, "hg summary" may fail, when there is inconsistent rebase state: for example, the root of rebase destination revisions recorded in rebase state file is already stripped manually. Mercurial earlier than 2.7 allows users to do anything other than starting new rebase, even though current rebase is not finished or aborted yet. So, such inconsistent rebase states may be left and forgotten in repositories. This patch catches RepoLookupError at restoring rebase state for summary hook, and treat such state as "broken".