minor `t/` fixups

I couldn't run the test scripts out of the box; these fixups let me at least run it (and it _looks_ like it all worked :) ).

Two other improvements I noticed while running the tests but didn't put into this series (because they are part of much larger issues):

  1. The `git-imerge` script points statically at `/usr/bin/python2`. On my Debian system, it's `/usr/bin/python`. If we want to allow both, I guess we'd need some kind of build infrastructure.

  2. The test scripts seem to want to be run in the current directory; but after running, you now have a .git directory in `t/`, which makes it hard to commit any improvements. Maybe I'm doing it wrong, but if not, doing it all in a trash directory would make sense. But of course that would probably be part of a larger test harness.