tests/test-update-renames.t

merge-tools: allow marking a mergetool as completely disabled

Very often in my life I'm finding that the only configured merge tool
present on the system is vimdiff[0], and it's currently impossible (as
far as I can tell) short of specifying `ui.merge = `[1] to actually
*disable* a merge tool. This allows vimdiff-haters to put:

  [merge-tools]
  vimdiff.disable = yes

in their ~/.hgrc and never see vimdiff again. I'm stopping short of
putting this as a commented out entry in the sample new user hgrc
(seen when a user runs `hg config --edit` with no ~/.hgrc) for now,
but I might come back and do that later.

0: vimdiff is at an awkward intersection: it's usually installed by
the vim package which is often installed as a vi substitute, so it's
mere presence doesn't imply me wanting it, unlike (say) kdiff3.

1: There's a related problem I ran into today where specifying
`ui.merge = :merge` failed because :merge isn't a command, which I
think is a regression. I'll try and figure that out and at least file
a bug.

Test update logic when there are renames

Update with local changes across a file rename

  $ hg init

  $ echo a > a
  $ hg add a
  $ hg ci -m a

  $ hg mv a b
  $ hg ci -m rename

  $ echo b > b
  $ hg ci -m change

  $ hg up -q 0

  $ echo c > a

  $ hg up
  merging a and b to b
  warning: conflicts while merging b! (edit, then use 'hg resolve --mark')
  0 files updated, 0 files merged, 0 files removed, 1 files unresolved
  use 'hg resolve' to retry unresolved file merges
  [1]