magit-git-dir: rename argument SUBDIR to PATH
magit-buffer-file-name: ensure returned value is full path

unless argument RELATIVE is non-nil, obviously.
magit-show: use full path for tempory  buffer-file-name value

`magit-show' temporarily binds `buffer-file-name' around the call to
`normal-mode' because that is needed to set directory local variables.

However it used the filename relative to the repository root.  Now that
is expanded to a full path before binding `buffer-file-name'.