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.

Ancestors of 5
4 2 0 
Ancestors of 6 and 5
3 4 2 1 0 
Ancestors of 5 and 4
4 2 0 

Descendants of 5
7 8 
Descendants of 5 and 3
6 7 8 
Descendants of 5 and 4
5 7 8