Context

This simple MR improves a bit the storage's UX to pave the way (and split into small MRs) to the integration of the irmin.3.7 release. It improves:

snapshot imports -- this is still not convincing, but it will be improved with irmin.3.7
context init, by adding an event

Manually testing the MR

Try to trigger the above ux changes

Document the interface of any function added or modified (see the coding guidelines)
Document any change to the user interface, including configuration parameters (see node configuration)
Provide automatic testing (see the testing guide).
For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
Select suitable reviewers using the Reviewers field below.
Select as Assignee the next person who should take action on that MR