When saving and restoring local state in Mercurial, also save and restore bookmarks

Summary:
Ref PHI1808. In Mercurial, we must save and restore bookmark state explicitly.

• Save and restore bookmarks.
• Clean up concepts in "arc-ls-markers" slightly, so we don't need separate "isCurrent" and "isActive" flags, hopefully.

I believe the totality of Mercurial state is:

• A (non-bare) working copy points at exactly one commit (which might be the empty/null commit, in an empty repository).
• A working copy has exactly one active branch.
  • Each branch has zero or more heads.
  • Each head may be closed.
  • Each (non-null) commit belongs to exactly one branch.
  • Note that the active branch may have zero heads and zero commits which belong to it!
• A working copy has zero or one active bookmark.

To capture this, we now emit:

• A list of branch heads. If a branch head is a working copy commit, that head is flagged as active.
• A list of bookmarks. If a bookmark is the current bookmark, that bookmark is flagged as active.
• A single "branch-state" virtual marker. This covers the case where you have run "hg branch X" to create X, but no objects in the working copy actually correspond to X yet. It also covers the case where you are on a concrete branch, but not any head of that branch.
• A single "commit-state" virtual marker. This always shows the current commit in the working copy.

Test Plan:

• Useful states to test are:
  • Empty repository (not all commands currently work here).
  • Normal repository, on a bookmark.
  • Normal repository, no bookmark.
  • "hg up 123" to update to somewhere in history.
  • "hg branch X", to start a new branch with no commits.
• Ran "arc branches" and "arc bookmarks" in various states. Saw generally sensible output.
• Ran "arc land --hold ..." in various states against a failing remote. Saw generally sensible output, and saw working properly restored to the original state.

Differential Revision: https://secure.phabricator.com/D21396