This is a simple hook for a centralized Git repository to which multiple users
have the ability to push.  If one user pushes a commit that they realize is a
mistake, adding the sha1 of that commit to the blacklist helps prevent other
users from unwittingly re-pushing the bad commit.

Prerequisites:

 * Perl - http://www.perl.org
 * DBM::Deep - http://search.cpan.org/dist/DBM-Deep/

Installation:

1) Copy the "update.blacklist" script to the hooks/ directory of the desired Git
repository, making sure that it is executable.  You should then either change
its name to "update", or have it execute from an existing update hook.  Remember
that the exit code of the hook is important, and a non-zero exit status should
be passed along from any wrapper.

2) Create a directory under hooks named data (hooks/data/).  You'll want to
ensure that this directory is writable by all users, and should probably have
0775 permissions.

3) Create a file in hooks/data/, again writable by all users, which is named
"blacklist".  You can use "blacklist.sample" as a template, which contains
some helpful comments for users.

4) Optionally create a "blacklist-error" file under hooks/data/ which instructs
the users how to resolve the error they received, or who to contact.  You may
want to use the included "blacklist-error.sample" file as an example message.

Usage:

When the hook is activated, it will read the "blacklist" file, and create a
blacklist.db file which contains any configuration found in that file.  If the
file exists already, the mtime of the blacklist file will be inspected to
determine if it must repopulate the database.