Auto merge of #4649 - jongiddy:gitignore-newline, r=alexcrichton

Add a newline before appended VCS ignore lines

The `cargo init` command appends to `.gitignore` or `.hgignore` without checking if the last line of the existing file ends with a newline.

This change will insert a newline before the appended lines. If the last line has no newline, this will ensure the lines are kept separated. If the newline is present, this will insert a blank line. Both Git and Mercurial ignore blank lines in these files.