Valgrind support: check for more than just programming errors

Valgrind support: check for more than just programming errors

This patch makes --valgrind try to override _all_ Git binaries in the
PATH, and it makes it an error to call *.sh and *.perl scripts directly.

While it is not strictly necessary to look through the whole PATH to
find git binaries to override, it is in line with running an expensive
test (which valgrind is) to make extra sure that only binaries are
tested that actually come from the git.git checkout.

In the same spirit, we can test that neither our test suite nor our
scripts try to run the *.sh or *.perl scripts directly.

It's more like a "because we can" than a "this is tightly connected
to valgrind", but in the author's opinion "because we can" is "so we
should" in this case.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>