This moves the git manpage to man7, since "git" isn't a direct command
per se. It also does two other things:
* Sort of works around the asciidoc 6.0.3 bug where the manpages all
get called "git.1". It just renames them to what they should have
been called.
* Fixes a cut-n-paste bug in git-diff-helper.txt that was making
asciidoc choke.