obsolete: update tests to use obsolete options The obsolete._enabled flag has become a config option. This updates all but one of the tests to use the minimal number of flags necessary for them to pass. For most tests this is just 'createmarkers', for a couple tests it's 'allowunstable', and for even fewer it's 'exchange'.

obsolete: prevent options from being used without createmarkers exchange and allowunstable should only be enabled if createmarkers is enabled, so check for that and raise an exception if that's not the case.

obsolete: add exchange option This adds an option that enables obsolete marker exchange.

obsolete: add allowunstable option This option allows the creation of unstable commits. This allows things like amending in the middle of a stack of commits, etc.

obsolete: add createmarkers option The basic obsolete option is allowing the creation of obsolete markers. This does not enable other features, such as allowing unstable commits or exchanging obsolete markers.

obsolete: add readonly flag to obstore constructor Previously, obstore read the obsolete._enabled flag to determine whether to allow writes to the obstore. Since obsolete._enabled will be moving into a repo specific config, we can't read it globally, and therefore must pass the information into the constructor.

obsolete: add isenabled function for option checking Previously, obsolete used the module level _enabled flag to determine whether it was on or off. We need a bit more granular control, so we'll be introducing toggle options. The isenabled() function is how you check if a particular option is enabled for the given repository. Future patches will add options such as 'createmarkers', 'allowunstable', and 'exchange' to enable various features of obsolete markers.