Put this in .git/hooks/commit-msg for each git project

gistfile1.pl

#!/usr/bin/env perl -w

my $file = $ARGV[0];
my $bad = 0;

open(my $FH, "<", $file);

while(<$FH>) {
    if ((length) > 50) {
        print("line $. too long\n");
        $bad = 1;
    }
}

$bad && exit 1