dabbrev-select-buffers-function project aware.

[Ergus]

Hi:

I am trying to limit dabbrev to know about current project context qhen
there is project-root.

 I am trying to implement a function to substitute
`dabbrev--same-major-mode-p`, to add an extra condition to check if both
buffers are in the same project. But some time in the past I reported
some performance issue that was fixed, but I am wondering if this
addition may worst instead of improve performance for dabbrev
completion.

Do we recommend anything special?

Best,
Ergus