switch all instances of 'git ...' to @git ...@ for consistent formatting

This patch fixes each instance of `'git ...'` within `text/*.textile` to use the pre-formatted style `@git ...@` instead.

Some but not all instances of git commands were using the `@...@` form already.

This patch is required if issue GH-6 (disable em-dash conversion within @code blocks@ in the PDF) is to be resolved, since the `'git --option'` form would still suffer from unwanted em-dashes.