tests: add the commit style checker from Mercurial

obscache: update the cache key in place Instead of computing the key in place, we update the existing one using the data used for the incremental update of the content. This will help reaching purely incremental cache eventually. The 'getcachekey' function is dropped as it is no longer used.

obscache: distinct 'clear' and 'reset' We make a difference between basic invalidatiton (or lack of loading) and 'reset' (current cache data are invalid). We can now ensure a non-None key when we start the update. This will allow us to "update" the existing key instead of recomputing one from scratch. This is more efficient and less racy.

merge with stable

serveronly: also enable the obscache for server only setting The obscache is useful and speeds things up. So we also enable for server only setting.

obscache: document a possible way forward to skipping obsstore parsing We document a possibly viable idea for updating the cache on new revs without having to fully parse the obsstore.

obscache: update some documentation Update some data and do some proof reading.

obscache: Only access the new obsmarkers for marker update Since we know what what is the part of the file with "new" markers we can just access these. If the cache was up to date before the transaction, we'll re-parse data we just wrote. See the inline comment for details. This is good enough for now.

obscache: extract _updatemarkers code into its own function This split the update logic from the bit retrieving markers. That cannot be bad.

obscache: extract code to update from new revision Code cleanup and prepare upstreaming. The make the code more likely to work with the key validation returning an iterator of revs to update the cache with.