Something like this should work:
#!/bin/sh tmpfile="$(mktemp)" sed "s|$2|###PASSWORDLINE###|" < "$1" > "$tmpfile" patchfile="$(mktemp)" diff "$tmpfile" "$1" > "$patchfile" sensible-editor "$tmpfile" patch -o "$1" "$tmpfile" "$patchfile" rm "$patchfile" rm "$tmpfile"