Use $GITPERLLIB instead of $RUNNING_GIT_TESTS and centralize @INC munging

Use $GITPERLLIB instead of $RUNNING_GIT_TESTS and centralize @INC munging

This makes the Git perl scripts check $GITPERLLIB instead of
$RUNNING_GIT_TESTS, which makes more sense if you are setting up your shell
environment to use a non-installed Git instance.

It also weeds out the @INC munging from the individual scripts and makes
Makefile add it during the .perl files processing, so that we can change
just a single place when we modify this shared logic. It looks ugly in the
scripts, too. ;-)

And instead of doing arcane things with the @INC array, we just do 'use lib'
instead, which is essentialy the same thing anyway.

I first want to do three separate patches but it turned out that it's quite
a lot neater when bundled together, so I hope it's ok.

Signed-off-by: Petr Baudis <pasky@suse.cz>
Signed-off-by: Junio C Hamano <junkio@cox.net>