rebase: allow aborting if last-message.txt is missing Previously, if .hg/rebasestate existed but .hg/last-message.txt was missing, 'hg rebase --abort' would say there's no rebase in progress but 'hg checkout foo' would say 'abort: rebase in progress'. It turns out loading the collapse message will throw a "no rebase in progress" error if the file doesn't exist, even though .hg/rebasestate obviously indicates a rebase is in progress. The fix is to only throw an exception if we're trying to --continue, and to just eat the issues if we're doing --abort. This issue is exposed by us writing the rebase state earlier in the process. This will be used by later patches to ensure the user can appropriately 'hg rebase --abort' if there's a crash before the first the first commit has finished rebasing. Tests cover all of this. The only negative affect is we now require a hg rebase --abort in a very specific exception case, as shown in the test.