tests/test-empty.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.

Create an empty repo:

  $ hg init a
  $ cd a

Try some commands:

  $ hg log
  $ hg grep wah
  [1]
  $ hg manifest
  $ hg verify
  checking changesets
  checking manifests
  crosschecking files in changesets and manifests
  checking files
  0 files, 0 changesets, 0 total revisions

Check the basic files created:

  $ ls .hg
  00changelog.i
  requires
  store

Should be empty:

  $ ls .hg/store

Poke at a clone:

  $ cd ..
  $ hg clone a b
  updating to branch default
  0 files updated, 0 files merged, 0 files removed, 0 files unresolved
  $ cd b
  $ hg verify
  checking changesets
  checking manifests
  crosschecking files in changesets and manifests
  checking files
  0 files, 0 changesets, 0 total revisions
  $ ls .hg
  00changelog.i
  hgrc
  requires
  store

Should be empty:

  $ ls .hg/store

  $ cd ..