rc mail: enable jumping from inline diff to source file
rc mail: enable jumping from inline diff to source file

This allows to jump from a mail buffer that contains an inline diff
to the source files (most accurate when the patch has been applied
locally).

This makes the diff module a mandatory dependency; we could relax that.

jkonecny12/kakoune,casimir/kakoune,jkonecny12/kakoune,jkonecny12/kakoune,casimir/kakoune,mawww/kakoune,jkonecny12/kakoune,mawww/kakoune,mawww/kakoune,casimir/kakoune,mawww/kakoune,casimir/kakoune