Merge in external changes made while editing

Question

If there are external changes made to a file being edited in Vim I understand that I can reload the content of the file by :edit. But, assume that (internally) I've made changes to the file that I want to preserve. Is there a way to integrate the two sets of changes (external and internal)? From what I understand, :edit simply takes the external changes.

Answer 1

To elaborate on the Christian Brabandt's comment, here is a decomposition of the DiffOrig command:

Full command:

command DiffOrig vert new | set bt=nofile | r # | 0d_ | diffthis
    \ | wincmd p | diffthis

Decomposition:

command DiffOrig        " create a new command named DiffOrig
vert new                " open a new buffer splitted vertically
| set bt=nofile         " set buffer type to scratch 
| r #                   " read the alternate buffer (opened file to diff)
| 0d_                   " delete the first line
| diffthis              " diff the current buffer
| wincmd p              " move cursor to opened file 
| diffthis              " diff the current buffer

An alternate version use the ++edit option for the read, it keeps the options values as if editing a file.

References:

• :h vert
• :h 'bt'
• :h read
• :h ++edit
• :h diffthis
• :h wincmd