install.sh: Fix some filename issues

There was a missing .zip file extension and also the commit-msg hook
needs some special care.

Signed-off-by: Ondrej Kupka <ondra.cap@gmail.com>
feature start: Install commit-msg

The required commit-msg hook is installed if absent and the current one
is replaced when it's not the same as the currently installed one. But
only if the user confirms that.

Signed-off-by: Ondrej Kupka <ondra.cap@gmail.com>
Merge pull request #1 from tchap/fix-install-script

Implement commit-msg hook handling