`git diff` doesn't perform EOL conversion on stdin, causing it to
mistakenly flag every line as having changed when the working tree uses
a different EOL than the blobs. Writing the buffer to a temporary file
and diffing against that avoids this issue.
Fixes#232.
Adds support for when a symlink file <target> points to another file
<source>, where <srouce> is a file under Git version control.
Thanks to @Z1MM32M4N for work on this (see #209).
Also don't pass buffer number to functions when they can look it up
themselves.
Using buffer numbers also eliminates any ambiguity which might arise
from symbolic links, where you have potentially two names for a file.
Thanks to @Z1MM32M4N for work on this (see #209).
