transaction: document `tr.add`

transaction: drop backupentries logic from startgroup and endgroup The `startgroup` and `endgroup` methods are used in a very specific context to wrap a very specific operation (revlog truncation). It does not make sense to perform any other operations during such a "group" (eg:file backup). There is currently no user of backupfile during a "group" so we drop the group-specific code and restrict authorized operations during "group".

transaction: document startgroup and endgroup These enigmatic methods are only used in repair. We document them to clarify there purpose and user.

transaction: mark backup-related attributes private As the transaction is gaining more functions and attributes, it is important to clarify what is part of the public API.

transaction: document the contents of `tr.backupentries` Now that all items are known we can document it.

transaction: drop the third item in `tr.backupentries` This third item is always None and never used.

rebase: fix rebase with no common ancestors (issue4446) The new rebase revset didn't check for the case when there are no common ancestors. Now it does. The new behavior should be the same as the pre-3.2 behavior. Added a test.

test-run-tests: accept more levels of precision and trailing ws (issue4440) simplejson produces slightly different output from the built-in json module, specifically: * It uses 0.000 instead of 0.0000 * It likes to put a trailing space after a comma This change works around both of those variations.

merge with stable

changegroup.cg2packer: lookup 'group' via inheritance chain This lets extensions insert themselves in the class hierarchy.