[PATCH 0 of 4 phases] secret changeset creation and exchange

[Pierre-Yves David]

Here is a resent of patch 4 and 6 with the ui option implementation.

First patch is a small doc update independent from 3 others patches.

I was not very happy with adding the secret argument to commitctx and I'm glad
to replace it with a ui option. This options is named "phases.new-commit".
While writing this down I'm thinking "phase.min-commit-phase" might be better.
What do you think ?

However I fell like a secret switch on commit are still pretty handy (based on
my experimentation of replacing mq with obsolete). If there is a clear
opposition to such switch, patch 3 may be ignored if "--secret" switch in tests
are replaced by proper --config call. Test update from patch 3 should be
applied in all case.

mq will mark imported changeset at secret anyway. This is to be implemented in
a later series.

Immutability related patch is sent in another series