Filemerge: Environment Variable HGMERGE uses merge tool pathfromtheconfiguration

[simohe at besonet.ch]

Mpm, thanks for discussing on irc. I regret that I had to leave because of
the time shift.
I see the problem when HGMERGE is not set to a simple command name. Some of
the tests of the test suite failed.
I send a new version including the changes which base on this change.

>
> What's this for? HGMERGE may not be a simple command name and we've
> always just called out to HGMERGE as supplied.
>