teach gitconfig-mode about .gitmodules
gitattributes-mode.el: improve Commentary

Also drop Created header keyword.
gitattributes-mode.el: collect all requires in one place
gitattributes-mode.el: cosmetics
gitattributes-mode.el: add myself as maintainer
gitconfig-mode.el: improve auto-mode-alist entries

When appropriate make sure file names begin with a dot "\\." not any
character ".".  Also stop using rx for these regexps.  I would like to
completely stop using it, but the other regexps are more difficult to
translate into something human readable.
gitconfig-{line-indented,point-in-indentation}-p: shorten doc-string

Use the same form built-in doc-strings use.
git-rebase-show-commit: fix use of magit-show-commit
git-rebase-mode.el: don't use rx
git-rebase-mode-font-lock-keywords: use backquote
gitignore-mode.el: don't use rx
gitignore-mode-font-lock-keywords: don't use rx

And fix the bugs along the way.
gitignore-mode-font-lock-keywords: fix recent bug
git-rebase-dead-line-re: fix recent bug
git-commit-abort: support Emacs < 24.1
Add git-commit-without-user-email custom variable

Fixes magit/magit#1098.