fix 526e9eb8dee4f3c220c4c9435cb1157ed1e506d3
magit-wash-hunk: parse stored heading as value
magit-visit-file: reuse existing window

When the file to be switched to is already visible in another
window of the current frame, then pop to that instead.
magit-hunk-file-position: new function

replacing `magit-hunk-target-line'.
magit-dired-jump: don't require dired-x

`dired-jump' is autoloaded, so we don't
need to explicitly require it before use.
magit-goto-section-hook: new variable
magit-{section,hunk}-set-window-start: new functions

Add `magit-hunk-set-window-start' to `magit-goto-section-hook'.
Maybe these functions are good enough, or maybe we need something
more elaborate.