add tests for merge-index / merge-one-file

add tests for merge-index / merge-one-file

There were no tests for either, except a brief use in
t1200-tutorial.

These tools are not used much these days, as most people
use the merge-recursive strategy, which handles everything
internally. However, they are used by the "octopus" and
"resolve" strategies, as well as any custom strategies
or merge scripts people have built around them.

For example, together with read-tree, they are the simplest
way to do a basic content-level merge without checking out
the entire repository contents beforehand.

This script adds a basic test of the tools to perform one
content-level merge. It also shows a failure of the tools to
work properly in the face of GIT_WORK_TREE or core.worktree.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>