GIT-BUILD-OPTIONS: propagate project's source directory

GIT-BUILD-OPTIONS: propagate project's source directory

A couple of our tests require knowledge around where to find the
project's source directory in order to locate files required for the
test itself. Until now we have been wiring these up ad-hoc via new,
specialized variables catered to the specific usecase. This is quite
awkward though, as every test that potentially needs to locate paths
relative to the source directory needs to grow another variable.

Introduce a new "GIT_SOURCE_DIR" variable into GIT-BUILD-OPTIONS to stop
this proliferation. Remove existing variables that can be derived from
it.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>