tests/svn/encoding.svndump

effectflag: store an empty effect flag for the moment

The idea behind effect flag is to store additional information in obs-markers
about what changed between a changeset and its successor(s). It's a low-level
information that comes without guarantees.

This information can be computed a posteriori, but only if we have all
changesets locally. This is not the case with distributed workflows where you
work with several people or on several computers (eg: laptop + build server).

Storing the effect-flag as a bitfield has several advantages:

- It's compact, we are using one byte per obs-marker at most for the effect-
  flag.
- It's compoundable, the obsfate log approach needs to display evolve history
  that could spans several obs-markers. Computing the effect-flag between a
  changeset and its grand-grand-grand-successor is simple thanks to the
  bitfield.

The effect-flag design has also some limitations:

- Evolving a changeset and reverting these changes just after would lead to
  two obs-markers with the same effect-flag without information that the first
  and third changesets are the same.

The effect-flag current design is a trade-off between compactness and
usefulness.

Storing this information helps commands to display a more complete and
understandable evolve history. For example, obslog (an Evolve command) use it
to improve its output:

x  62206adfd571 (34302) obscache: skip updating outdated obscache...
|    rewritten(parent) by Matthieu Laneuville <matthieu.laneuville@octobus...
|    rewritten(content) by Boris Feld <[e-mail]>

The effect flag is stored in obs-markers metadata while we iterate on the
information we want to store. We plan to extend the existing obsmarkers
bit-field when the effect flag design will be stabilized.

It's different from the CommitCustody concept, effect-flag are not signed and
can be forged. It's also different from the operation metadata as the command
name (for example: amend) could alter a changeset in different ways (changing
the content with hg amend, changing the description with hg amend -e, changing
the user with hg amend -U). Also it's compatible with every custom command
that writes obs-markers without needing to be updated.

The effect-flag is placed behind an experimental flag set to off by default.

Hook the saving of effect flag in create markers, but store only an empty one
for the moment, I will refine the values in effect flag in following patches.

For more information, see:

  https://www.mercurial-scm.org/wiki/ChangesetEvolutionDevel#Record_types_of_operation

Differential Revision: https://phab.mercurial-scm.org/D533

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