[PATCH] git-mergetool--lib.sh: fix mergetool.<tool>.* configurations ignored for known tools

Hi,

Here a patch I made after Junio's remarks in this thread :
http://thread.gmane.org/gmane.comp.version-control.git/148267

I think git-mergetool--lib.sh needs some refactoring. I'm no sh expert,
far from it, but I know a bit about scripting (I have been developing in PHP
for more than 6 years now :P) and I had quite some difficulties to understand
the behavior of some functions. For example, get_merge_tool_path returns the
name of the tool if no mergetool.<tool>.path have been set, which, from my
point of view, makes no sense.

I know there is maybe no time or no need to rewrite something which works
rather well but the +5 hours I spent to write this poor patch make me 
wonder if I am overrating myself. So if someone could just agree with me   
that would be a huge step to help me to regain some self respect :)

Cheers.