Add valgrind support in test scripts

Add valgrind support in test scripts

This patch adds the ability to use valgrind's memcheck tool to
diagnose memory problems in Git while running the test scripts.

It requires valgrind 3.4.0 or newer.

It works by creating symlinks to a valgrind script, which have the same
name as our Git binaries, and then putting that directory in front of
the test script's PATH as well as set GIT_EXEC_PATH to that directory.
Git scripts are symlinked from that directory directly.

That way, Git binaries called by Git scripts are valgrinded, too.

Valgrind can be used by specifying "GIT_TEST_OPTS=--valgrind" in the
make invocation. Any invocation of git that finds any errors under
valgrind will exit with failure code 126. Any valgrind output will go
to the usual stderr channel for tests (i.e., /dev/null, unless -v has
been specified).

If you need to pass options to valgrind -- you might want to run
another tool than memcheck, for example -- you can set the environment
variable GIT_VALGRIND_OPTIONS.

A few default suppressions are included, since libz seems to trigger
quite a few false positives. We'll assume that libz works and that we
can ignore any errors which are reported there.

Note: it is safe to run the valgrind tests in parallel, as the links in
t/valgrind/bin/ are created using proper locking.

Initial patch and all the hard work by Jeff King.

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