bug#41974: 28.0.50; Some vc commands refuse to work when called on non-file buffer

[Juri Linkov]

For example, typing

  M-x vc-print-branch-log RET

on the project's *compilation* buffer signals the error:

  "Buffer *compilation* is not associated with a file"

I think this restriction should be lifted, so vc commands should
work even when called on non-file buffers whose default-directory
is inside the vc project.