localrepo: make report level in repo.transaction configurable

repo.transaction always writes to stderr when a transaction aborts. In order to
be able to abort a transaction quietly (e.g shelve needs a temporary view on
the repo) we need to make the report level configurable.
shelve: add a shelve extension to save/restore working changes

This extension saves shelved changes using a temporary draft commit,
and bundles the temporary commit and its draft ancestors, then
strips them.

This strategy makes it possible to use Mercurial's bundle and merge
machinery to resolve conflicts if necessary when unshelving, even
when the destination commit or its ancestors have been amended,
squashed, or evolved. (Once a change has been unshelved, its
associated unbundled commits are either rolled back or stripped.)

Storing the shelved change as a bundle also avoids the difficulty
that hidden commits would cause, of making it impossible to amend
the parent if it is a draft commits (a common scenario).

Although this extension shares its name and some functionality with
the third party hgshelve extension, it has little else in common.
Notably, the hgshelve extension shelves changes as unified diffs,
which makes conflict resolution a matter of finding .rej files and
conflict markers, and cleaning up the mess by hand.

We do not yet allow hunk-level choosing of changes to record.
Compared to the hgshelve extension, this is a small regression in
usability, but we hope to integrate that at a later point, once the
record machinery becomes more reusable and robust.
shelve: new output format for shelve listings

Use a more condensed and mercurial-like output format for shelve listing.
We don't prefix the message with 'shelved from...' anymore as our default
name contains the branch name or the user used his own name. To avoid
just printing the last commit message, we drop writing the description
to stdout.

old output:
  default         [1s ago]    shelved from default (01ba9745): create conflict

new output:
  default         (1s ago)    create conflict
shelve: allow shelving of a change with an mq patch applied

We allow shelving of of changes on top of a MQ repository. MQ will
not allow repository changes on top of applied patches. We introduce
checkapplied in MQ to bypass this check.