util: add a helper class to compute digests It is going to be used for the remote-changegroup feature in bundle2.

bundle2: merge return values when bundle contains multiple changegroups A bundle2 may contain multiple parts adding changegroups, in which case there are multiple operation records for changegroups, each with its own return value. Those multiple return values are aggregated in a single cgresult value for the whole operation. As can be seen in the associated test case, the situation with hooks is not really the best, but without deeper thoughts and changes, we can't do much better. Hopefully, things will be improved before bundle2 is enabled by default. In the meanwhile, multiple changegroups is not expected to be in widespread use, and even less expected to be used for pushes. Also, not many clients cloning or pulling bundle2 with multiple changesets are not expected to have changegroup hooks anyways.

changegroup: use a copy of hookargs when invoking the changegroup hook addchangegroup creates a runhook function that is used to invoke the changegroup and incoming hooks, but at the time the function is called, the contents of hookargs associated with the transaction may have been modified externally. For instance, bundle2 code affects it with obsolescence markers and bookmarks info. It also creates problems when a single transaction is used with multiple changegroups added (as per an upcoming change), whereby the contents of hookargs are that of after adding a latter changegroup when invoking the hook for the first changegroup.

tests: pull common http server setup out of individual tests There are currently two different tests using roughly the same code to create temporary scripts acting as HTTP servers. As there is going to be at least one more in an upcoming change, factor those out in a standalone dumbhttp.py script.