Code review comment for lp:~doxxx/bzr/mergetools

I've made more changes according to vila's comments on https://code.launchpad.net/~doxxx/bzr/mergetools-commands/+merge/40924.

I'm not 100% sure I've got the treatment of known merge tools right. One problem is that with my modifications to qconfig, it now shows all the user-defined and pre-defined merge tools, and if the user clicks Okay, it's going to save all of them as user-defined, which negates the usefulness of having the pre-defined merge tools as vila described in the other mp. qconfig needs to be aware of the difference between a user-defined and a pre-defined merge tool. But at the same time, I don't want to make it hard for clients of the mergetools module and its functions on Config to use user-defined or pre-defined mergetools.