Improve modi/copy-buffer-file-name
Improve modi/copy-buffer-file-name

Earlier, this command was named `show-copy-buffer-file-name`. It is now
renamed to `modi/copy-buffer-file-name'.

In addition to renaming, also optimized the code:
- Pulled file-name variable assignment into the let*
- Make use of file-name-nondirectory
- Improve the displayed message
- Update the command doc-string

kaushalmodi/.emacs.d,kaushalmodi/.emacs.d