use internal merge tool when specified for a merge-pattern in hgrc

It is possible to specify in the hgrc file that one of the internal merge
tools (internal:fail, internal:local or internal:other) be used for
performing merges on files matching a given pattern. However, this setting
is not being acted upon (the merge tool is not found). This patch fixes that.

mercurial/filemerge.py

@@ -16,6 +16,8 b' def _toolbool(ui, tool, part, default=Fa'
     return ui.configbool("merge-tools", tool + "." + part, default)
 
 def _findtool(ui, tool):
+    if tool in ("internal:fail", "internal:local", "internal:other"):
+        return tool
     k = _toolstr(ui, tool, "regkey")
     if k:
         p = util.lookup_reg(k, _toolstr(ui, tool, "regname"))