Add a Makefile to run subtree tests. This is largely copied
from the standard test suite with irrelevant targets removed
and some paths altered to account for where subtree tests live.
Signed-off-by: David A. Greene <greened@obbligato.org>
Set TEST_DIRECTORY to the main git test area. This allows the
git-subtree out-of-tree tests to run correctly.
Signed-off-by: David A. Greene <greened@obbligato.org>