t/perf: do not run tests in user's $SHELL

t/perf: do not run tests in user's $SHELL

The environment variable $SHELL is usually set to the user's
interactive shell. Our build and test scripts never use $SHELL because
there are no guarantees about its input language.  Instead, we use
/bin/sh which should be a POSIX shell.

For systems with a broken /bin/sh, we allow to override that path via
SHELL_PATH.  To run tests in yet another shell we allow to override
SHELL_PATH with TEST_SHELL_PATH.

Perf tests run in $SHELL via a wrapper defined in t/perf/perf-lib.sh,
so they break with e.g. SHELL=python.  Use TEST_SHELL_PATH like
in other tests.  TEST_SHELL_PATH is always defined because
t/perf/perf-lib.sh includes t/test-lib.sh, which includes
GIT-BUILD-OPTIONS.

Acked-by: Jeff King <peff@peff.net>
Signed-off-by: Johannes Altmanninger <aclopte@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>