extdiff: don't run gui programs when in a cli-only environment...
extdiff: don't run gui programs when in a cli-only environment
In order to provide a useful error message to override the behavior, we also
need to slightly change the way that tool.gui is found in the config. Before,
it had to be where tool.diffargs is located, which might not exist. Now,
tool.isgui can exist on its own.
A test is added for the new error message. We also need to force
procutil.isgui() to return true, so we set $DISPLAY to a non-empty value
before running any test expecting to have a gui.
