magit-rebase-step: cosmetics
add new function magit-file-line
magit-rebase-info: cosmetics

While this commit by itself looks like I might be going from one
extrem (common the the Magit source) of having very descriptive variable
names to having one letter names I do not have any intentions to do this
everywhere.  Here it makes sense though: the meaning is still obvious
and it helps a lot making the code cleaner.
magit-status-refresh: show commit rebase stopped at

Suggested-By: Moritz Bunkus <moritz@bunkus.org>