gfm-mode (which is part of markdown-mode.el). From the commentary in the file:
;;; GitHub Flavored Markdown:
;; A [GitHub Flavored Markdown][GFM] (GFM) mode, `gfm-mode', is also
;; available. The GitHub implementation of differs slightly from
;; standard Markdown. The most important differences are that
;; newlines are significant, triggering hard line breaks, and that
;; underscores inside of words (e.g., variable names) need not be
;; escaped. As such, `gfm-mode' turns off `auto-fill-mode' and turns
;; on `visual-line-mode' (or `longlines-mode' if `visual-line-mode' is
;; not available). Underscores inside of words (such as
;; test_variable) will not trigger emphasis.
;; Wiki links in this mode will be treated as on GitHub, with hyphens
;; replacing spaces in filenames and where the first letter of the
;; filename capitalized. For example, `[[wiki link]]' will map to a
;; file named `Wiki-link` with the same extension as the current file.
;; GFM code blocks, with optional programming language keywords, will
;; be highlighted. They can be inserted with `C-c C-s P`. If there
;; is an active region, the text in the region will be placed inside
;; the code block. You will be prompted for the name of the language,
;; but may press enter to continue without naming a language.