Some test scripts run Perl scripts as if they were git-* scripts, and
thus need to use the same perl that will be put in the shebang line of
git*.perl commands. $PERL_PATH therefore needs to be used instead of
a bare "perl".

The tests can fail if another perl is found in $PATH before the one
defined in $PERL_PATH.

Example test failure caused by this: the perl defined in $PERL_PATH has
Error.pm installed, and therefore the Git.pm's Makefile.PL doesn't install
the private copy. The perl from $PATH doesn't have Error.pm installed, and
all git*.perl scripts invoked during the test will fail loading Error.pm.

Makefile patch by Jeff King <peff@peff.net>.
Signed-off-by: NPhilippe Bruhat (BooK) <book@cpan.org>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>