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.

mercurial/filemerge.py

@@ -120,12 +120,15 b' def _picktool(repo, ui, path, binary, sy'
 
     # then merge tools
     tools = {}
+    disabled = set()
     for k, v in ui.configitems("merge-tools"):
         t = k.split('.')[0]
         if t not in tools:
             tools[t] = int(_toolstr(ui, t, "priority", "0"))
+        if _toolbool(ui, t, "disabled", False):
+            disabled.add(t)
     names = tools.keys()
-    tools = sorted([(-p, t) for t, p in tools.items()])
+    tools = sorted([(-p, t) for t, p in tools.items() if t not in disabled])
     uimerge = ui.config("ui", "merge")
     if uimerge:
         if uimerge not in names:

mercurial/help/config.txt

@@ -1004,7 +1004,10 b' Example ``~/.hgrc``::'
   kdiff3.priority = 1
 
   # Changing the priority of preconfigured tool
-  vimdiff.priority = 0
+  meld.priority = 0
+
+  # Disable a preconfigured tool
+  vimdiff.disabled = yes
 
   # Define new tool
   myHtmlTool.args = -m $local $other $base $output

tests/test-merge-tools.t

@@ -193,6 +193,26 b' or false set higher on command line:'
   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