branching: merge default into stable for 6.8rc0

relnotes: add 6.8rc0

branch3: use an explicitely experimental name for files Since this format is still experimental, we don't want to have to side-step the `branch3` name in case people do start using it before it's stable.

obsolete: simplify relevantmarker Drop duplicate assignment from a merge failure. Save one loop iteration by exploiting that pendingnodes will be seennodes after the first round anyway, so just pre-initialize the set accordingly. From Anton Shestakov's review on !867. Performance difference for my test case is in the noise.

exchange: improve computation of relevant markers for large repos Find the candidates for nodes with relevant markers by going over all markers instead of iterating over all nodes. Most nodes will not have markers anyway. Further optimize the code by allowing revsets as well, which reduces the materialization cost.

test: better glob some timing related line to avoid flakiness If we go over 10 seconds, the number of white space changes.

branching: merge stable into default

Added signature for changeset a1a011d4b148

Added tag 6.7.4 for changeset a1a011d4b148

relnotes: add 6.7.4 and warn about 6.7.{1,2,3}