Introduce variables GIT_TEST_INSTALLED and GIT_TEST_EXEC_PATH such that
the test suite can be run against a git which is installed at
GIT_TEST_INSTALLED with subcommands at GIT_TEST_EXEC_PATH.

GIT_TEST_INSTALLED defaults to the git.git checkout, GIT_TEST_EXEC_PATH
defaults to the output of '$GIT_TEST_INSTALLED/git --exec-path'.

Run the suite e.g. as

    GIT_TEST_INSTALLED=/some/path make test

but note that this requires and uses parts of a compiled git in the
git.git checkout: test helpers, templates and perl libraries are taken
from there.
Signed-off-by: NMichael J Gruber <git@drmicha.warpmail.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>