`tabulated-list-widen-current-column' previously looked only at the
width of the column as given in `tabulated-list-format' however this
fails to account for both the padding and content length and as a
result widens the wrong column.

The issue with content length is reproducible as follows:
0. emacs -Q
1. M-x shell
2. M-x list-processes
3. C-x o
4. C-: (tabulated-list-set-col 0 (make-string 50 ?.) t)
5. M-f
6. }


This patch should solve the issue however it relies on
`tabulated-list-get-entry' which means it fails to work if the
`tabulated-list-entry' property is not set properly.  I am unsure if
this should be considered a bug on the tabulated-list end or not.