Missing diff-mode dependency for faces

[aathn]

The envrc--summarise-changes function uses faces diff-changed, diff-added, and diff-removed, but envrc does not require diff-mode. This causes messages upon startup of the form

Invalid face reference: diff-changed [2 times]
Invalid face reference: diff-added
Invalid face reference: diff-changed [2 times]
...

if diff-mode was not previously loaded (on Emacs 29.1).

[purcell]

Thanks, I pushed a fix.