aggregate.perl did not work when Git.pm is not installed to a directory
contained in the default Perl library path list or PERLLIB.
This commit prepends the Perl library path of the current Git source
tree to enable this.

Note that this commit adds a hard-coded relative path

  use lib '../../perl/blib/lib';

instead of the flexible environment-based variant

  use lib (split(/:/, $ENV{GITPERLLIB}));

which is used in tests written in Perl.
The hard-coded variant is used because the whole performance test
framework does it that way (and GITPERLLIB is not set there).
Signed-off-by: NStephan Beyer <s-beyer@gmx.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>