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.

--- a/tests/test-merge-tools.t	Tue Oct 13 23:04:53 2015 -0700
+++ b/tests/test-merge-tools.t	Wed Oct 14 12:57:33 2015 -0400
@@ -193,6 +193,26 @@
   M f
   ? f.orig
 
+or true set to disabled:
+  $ beforemerge
+  [merge-tools]
+  false.whatever=
+  true.priority=1
+  # hg update -C 1
+  $ hg merge -r 2 --config merge-tools.true.disabled=yes
+  merging f
+  merging f failed!
+  0 files updated, 0 files merged, 0 files removed, 1 files unresolved
+  use 'hg resolve' to retry unresolved file merges or 'hg update -C .' to abandon
+  [1]
+  $ aftermerge
+  # cat f
+  revision 1
+  space
+  # hg stat
+  M f
+  ? f.orig
+
 or true.executable not found in PATH:
 
   $ beforemerge