Summary:
--retry can be used instead of --continue when histedit failed on "exec" (due to it returning a non-zero exit code) and the user wants to repeate that command rather than continuing past it
--show-plan will print the remaining plan of histedit during an interrupted histedit
Test Plan: cd fb-hgext/tests/ && rt
Reviewers: #fbhgext, stash
Reviewed By: #fbhgext, stash
Subscribers: stash, quark
Differential Revision: https://phab.mercurial-scm.org/D176
