fix git perl scripts in case of local::lib

I installed a new List::Util in my local::lib, but it was for Perl
5.16.2. Since Git defaults to /usr/bin/perl, which was 5.10.1, git could
not run its perl scripts:

    $ git add -i
    /usr/bin/perl: symbol lookup error: ~/perl5/lib/perl5/x86_64-linux-thread-multi/auto/List/Util/Util.so:
    undefined symbol: Perl_xs_apiversion_bootcheck
    fatal: 'add--interactive' appears to be a git command, but we were not
    able to execute it. Maybe git-add--interactive is broken?

All the git perl scripts seem to check GITPERLLIB envvar during startup,
so we can shove all of /usr/bin/perl's lib dirs on to the beginning of
@INC to override what's already there because of local::lib.

I feel `#!/usr/bin/env perl` should be used instead, but this thread
makes it seem like that was attempted and rejected:
http://article.gmane.org/gmane.comp.version-control.git/147462

I've tried setting PERL_PATH="/usr/bin/env perl" during git make,
according to http://article.gmane.org/gmane.comp.version-control.git/147475,
but this did not seem to fix the issue.