Require git v1.7.4.1.

Per the git v1.7.4.1 release notes at
https://github.com/git/git/blob/master/Documentation/RelNotes/1.7.4.1.txt):

		"git diff --cached HEAD" before the first commit does what an end user
		would expect (namely, show what would be committed without further "git
    add").

App::GitHooks::StagedChanges relies on being able to run a variation of this in
all situations, including right after initializing a repository. Since v1.7.4.1
was released on 2011-02-11, it looks safe to bump the git requirements to that
version.
Document git requirements.
Fix wording.
Release version 1.0.6.