tests/test-abort-checkin.t

copies: add config option for writing copy metadata to file and/or changset

This introduces a config option that lets you choose to write copy
metadata to the changeset extras instead of to filelog. There's also
an option to write it to both places. I imagine that may possibly be
useful when transitioning an existing repo.

The copy metadata is stored as two fields in extras: one for copies
since p1 and one for copies since p2.

I may need to add more information later in order to make copy tracing
faster. Specifically, I'm thinking out recording which files were
added or removed so that copies._chaincopies() doesn't have to look at
the manifest for that. But that would just be an optimization and that
can be added once we know if it's necessary.

I have also considered saving space by using replacing the destination
file path by an index into the "files" list, but that can also be
changed later (but before the feature is ready to release).

Differential Revision: https://phab.mercurial-scm.org/D6183

  $ cat > abortcommit.py <<EOF
  > from mercurial import error
  > def hook(**args):
  >     raise error.Abort(b"no commits allowed")
  > def reposetup(ui, repo):
  >     repo.ui.setconfig(b"hooks", b"pretxncommit.nocommits", hook)
  > EOF
  $ abspath=`pwd`/abortcommit.py

  $ cat <<EOF >> $HGRCPATH
  > [extensions]
  > mq =
  > abortcommit = $abspath
  > EOF

  $ hg init foo
  $ cd foo
  $ echo foo > foo
  $ hg add foo

mq may keep a reference to the repository so __del__ will not be
called and .hg/journal.dirstate will not be deleted:

  $ hg ci -m foo
  error: pretxncommit.nocommits hook failed: no commits allowed
  transaction abort!
  rollback completed
  abort: no commits allowed
  [255]
  $ hg ci -m foo
  error: pretxncommit.nocommits hook failed: no commits allowed
  transaction abort!
  rollback completed
  abort: no commits allowed
  [255]

  $ cd ..