Fix test scripts' use of directories.

The test scripts used to assume that they were being run from a
strange place.  Instead, teach them to derive the main project
directory "BASE" from $0 and

* always place the temporary directory at $BASE/t/tmp

* always run git-imerge from $BASE/git-imerge

* be more careful before deleting an old test directory.
Add a script to remove test detritus from the temporary test repository.
Rename "test-conflictless" to "test-unconflicted".

I have typed it wrong several times now so it seems to want its
new name.
Merge branch 'test-dirs'

Manage the directories used by the tests better.