simplemerge: support three labels when merging

If a third label is provided it will be used for the "base" content:

  <<<<<<< local
  content
  from
  local
  ||||||| base
  former
  common
  =======
  other
  conflicting
  >>>>>>> other

mercurial/simplemerge.py

@@ -379,13 +379,16 b' def simplemerge(ui, local, base, other, '
 
     name_a = local
     name_b = other
+    name_base = None
     labels = opts.get('label', [])
     if len(labels) > 0:
         name_a = labels[0]
     if len(labels) > 1:
         name_b = labels[1]
     if len(labels) > 2:
-        raise util.Abort(_("can only specify two labels."))
+        name_base = labels[2]
+    if len(labels) > 3:
+        raise util.Abort(_("can only specify three labels."))
 
     try:
         localtext = readfile(local)
@@ -402,7 +405,11 b' def simplemerge(ui, local, base, other, '
         out = sys.stdout
 
     m3 = Merge3Text(basetext, localtext, othertext)
-    for line in m3.merge_lines(name_a=name_a, name_b=name_b):
+    extrakwargs = {}
+    if name_base is not None:
+        extrakwargs['base_marker'] = '|||||||'
+        extrakwargs['name_base'] = name_base
+    for line in m3.merge_lines(name_a=name_a, name_b=name_b, **extrakwargs):
         out.write(line)
 
     if not opts.get('print'):

tests/test-contrib.t

@@ -184,10 +184,25 b' 2 labels'
   warning: conflicts during merge.
   [1]
 
+3 labels
+
+  $ python simplemerge -p -L foo -L bar -L base conflict-local base conflict-other
+  base
+  <<<<<<< foo
+  not other
+  end
+  ||||||| base
+  =======
+  other
+  end
+  >>>>>>> bar
+  warning: conflicts during merge.
+  [1]
+
 too many labels
 
-  $ python simplemerge -p -L foo -L bar -L baz conflict-local base conflict-other
-  abort: can only specify two labels.
+  $ python simplemerge -p -L foo -L bar -L baz -L buz conflict-local base conflict-other
+  abort: can only specify three labels.
   [255]
 
 binary file