cache: make the cache updated callback easily accessible to extension This will help extension to benefit from this new logic. As a side effect this clarify the 'transaction' method a little bit.

obscheckhead: skip context creation to test phase On repository with many many heads, this is called often enough to show an overhead to appears in some profile. So we skip the 'changectx' creation to test the node phases.

tests: add missing 'test-push-checkheads-partial-C1.t' For some reason, this one test case did not made it with the others as they got upstreamed during the 4.2 cycle.