Unify STDIN treatment for hooks

Four hooks get data from STDIN: pre-receive, post-receive, pre-push,
and post-rewrite. This commit unifies the way STDIN is grokked by
them.

The Git::More class has two new methods to make this data available:
push_input_data and get_input_data.

This way we do not need to fake extra arguments for the hooks, which
avoids compatibility problems in the future if some of them get extra
arguments from Git itself.

In particular, the pre-push hook, which was recently implemented,
doesn't get the faked 'push-specs' argument anymore.

Signed-off-by: Gustavo L. de M. Chaves <gnustavo@cpan.org>
Process STDIN for post-rewrite hook

The post-rewrite hook receives information from STDIN much like the
pre-push, pre-receive, and post-receive hooks. It wasn't being
processed by an oversight.

Signed-off-by: Gustavo L. de M. Chaves <gnustavo@cpan.org>