use build-time SHELL_PATH in test scripts

use build-time SHELL_PATH in test scripts

The top-level Makefile now creates a GIT-BUILD-OPTIONS file
which stores any options selected by the make process that
may be of use to further parts of the build process.
Specifically, we store the SHELL_PATH so that it can be used
by tests to construct shell scripts on the fly.

The format of the GIT-BUILD-OPTIONS file is Bourne shell,
and it is sourced by test-lib.sh; all tests can rely on just
having $SHELL_PATH correctly set in the environment.

The GIT-BUILD-OPTIONS file is written every time the
toplevel 'make' is invoked. Since the only users right now
are the test scripts, there's no drawback to updating its
timestamp. If something build-related depends on this, we
can do a trick similar to the one used by GIT-CFLAGS.

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