[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.