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.

rebase: add storestatus support for transactions This let's the status writing logic support transactions. This will be useful in a later patch where we add a transaction around the entire rebase.

rebase: move storestatus onto rebaseruntime The rebaseruntime class already has the restorestatus function, so let's make it own the store status function too. This get's rid of a lot of unnecessary argument passing and will make a future patch cleaner that refactors storestatus to support transactions.

rebase: clear updatestate during rebase --abort in more cases Previously, rebase --abort would only call update if you were on a node that had already been rebased. This meant that if the rebase failed during the rebase of the first commit, the working copy would be left dirty (with a .hg/updatestate file) and rebase --abort would not have update to clean it up. The fix is to also perform an update if you're still on the target node or on the original working copy node (since the working copy may be dirty, we still need to do the update). We don't want to perform an update in all cases though because of issue4009. A subsequent patch makes this case much more common, since it causes the entire rebase transaction to rollback during unexpected exceptions. This causes the existing test-rebase-abort.t to cover this case.