acme: delete selected text can delete extra characters not part of selection

[bhuntsman]

I found that in certain circumstances, acme can delete extra characters that are not part of the selected text.

To reproduce:

1. Create new acme window
2. Enter a string:
   /path/to/file
3. Double-click file. At this point, "file" will be highlighted, but not the / before file.
4. Press delete.
5. "/file" is deleted, leaving the string:
   /path/to

At first I thought this was by design, but I noticed that if I use the string:

pathtofile

and just highlight "file", and press delete, it also deletes "ofile" leaving "patht". This is clearly a bug and not a convenience for dealing with paths.

[bhuntsman]

This is a duplicate fo #424. Apologies.