Preserve marks when reading in new temporary files

Use :lockmarks to preserve marks in the file being read in.  Also use
:0r instead of :r (and delete the last line of the buffer rather than
the first, of course).  Also use ++edit on the read command, to allow
fileformat, encoding, BOM, etc detection to run.

Test case:
:e file://.vimrc
10G
mA
:enew
'A
" You should be on line 10 of the buffer.

Signed-off-by: Matt Wozniski <godlygeek@gmail.com>