Support pre-v1.6.5 gits by avoiding a directory argument to git-init

The git-init command begun to accept a directory argument on
v1.6.5. In order to support previous gits we chdir to the repo
directory, then we invoke git-init without an argument, and then we
chdir back to the original directory.
Consolidate methods Git::More::get_config and Git::More::config

The method Git::More::get_config accepted no arguments and returned a
two-level hash containing all configuration information.

The method Git::More::config accepted two arguments and returned the
values of a single configuration variable.

Now, the single method Git::More::get_config accepts zero, one, or two
arguments. With zero arguments it behaves exactly in the same way as
previously. With two arguments it behaves exactly as the no longer
existing Git::More::config method. With one argument it returns the
one-level hash with all configuration variables os a single
configuration section.

This change affected all plugins.

It was motivated by the fact that "config" is the name of a git
command. I'm evaluating other git wrappers to, perhaps, substitute
App::gh::Git and at least one of them (Git::Wrapper) uses AUTOLOAD to
make every git command a possible method. So, if I decide to use
Git::Wrapper or any other wrapper that uses the same technique, I
can't have a Git::More method with a name conflicting with a git
command. That's why I consolidated the methods with the "get_config"
name and not simply with "config".
Substitute Git for App::gh::Git

The Git.pm module is distributed with git and is not at CPAN yet. A
while back I decided to require App::gh::Git, which is a copy of Git
maintained by the GitHub people. However, I needed to require App::gh
instead of App::gh::Git directly, due to some versioning problems in
the latter, which isn't good, because App::gh carries some heavy
dependencies of its own.

In the end, I decided to get back to use Git directly. I can't depend
on it directly, but this way our dependencies are fewer and I have at
least the peace of mind of being using The Real Thing (c).
Try to find Git if it's not on @INC

Since I can't depend directly on Git I made Makefile.PL try to require
it to see if it can find it. If it can't, we don't install Git::Hooks
and produce a verbose message telling the user why and what she can do
about it.

Moreover, we allow the user to specify a directory for Git.pm using
the GITPERLLIB environment variable. This is inspired by the way the
git Perl scripts do it, but it's not quite the same. The git Perl
scripts prepend the directories in GITPERLLIB to @INC, making them
used to every use/require statement in the program, which can lead to
problems. In our case, we make a temporary change to @INC just in
order to require Git.pm.

If Makefile.PL can't require Git.pm directly, it tries to find it in
some usual places and inform the user before aborting the
installation.

Some of this was inspired by Randall Schwartz's post at:
http://git.661346.n2.nabble.com/better-way-to-find-Git-pm-officially-td7416362.html
v0.033

        Substitute Git for App::gh::Git as a parent for Git::More. The
        major motivation for this was to get rid of some heavy
        dependencies that were carried over by App::gh. This change
        should be mostly transparent, as App::gh::Git is a copy of
        Git.

        Since Git isn't on CPAN we can't depend directly on it. The
        user must guarantee that it is present on @INC in order to
        install and use Git::Hooks. The environment variable
        GITPERLLIB can be used to specify a list of directories
        (besides the ones already in @INC) where the module will be
        searched.

        Consolidate the methods Git::More::get_config and
        Git::More::config in a single method Git::More::get_config,
        which returns ever more specialized configuration depending on
        how many arguments it gets (zero, one, or two). This change
        affects most plugins.

        Make a change in the way we inkoke "git init" during test so
        that we can use pre-v1.6.5 gits.