filemerge: introduce a 'regkeyalt' merge tool variable

This allows us to provide alternate search keys for 64bit operating systems that
may have 32bit merge tools installed.  Presumably it may find other uses.

doc/hgrc.5.txt

@@ -556,6 +556,12 b' Supported arguments:'
   tool. Mercurial will search for this key first under
   ``HKEY_CURRENT_USER`` and then under ``HKEY_LOCAL_MACHINE``.
   Default: None
+``regkeyalt``
+  An alternate Windows registry key to try if the first key is not
+  found.  The alternate key uses the same ``regname`` and ``regappend``
+  semantics of the primary key.  The most common use for this key
+  is to search for 32bit applications on 64bit operating systems.
+  Default: None
 ``regname``
   Name of value to read from specified registry key. Defaults to the
   unnamed (default) value.

mercurial/filemerge.py

@@ -25,8 +25,10 b' def _toollist(ui, tool, part, default=[]'
 def _findtool(ui, tool):
     if tool in _internal:
         return tool
-    k = _toolstr(ui, tool, "regkey")
-    if k:
+    for kn in ("regkey", "regkeyalt"):
+        k = _toolstr(ui, tool, kn)
+        if not k:
+            continue
         p = util.lookup_reg(k, _toolstr(ui, tool, "regname"))
         if p:
             p = util.find_exe(p + _toolstr(ui, tool, "regappend"))