run test suite without dashed git-commands in PATH

run test suite without dashed git-commands in PATH

Only put bin-wrappers in the PATH (not GIT_EXEC_PATH), to emulate the
default installed user environment, and ensure all the programs run
correctly in such an environment.  This is now the default, although
it can be overridden with a --with-dashes test option when running
tests.

Signed-off-by: Matthew Ogilvie <mmogilvi_git@miniinfo.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>