xref call project-current in correct buffer

[Stephen Leake]

I ran into a situation where xref failed because
xref-show-definitions-buffer-at-bottom calls project-current in the
*xref* buffer, not the original source buffer, and the project in that
buffer is different from the one in the *xref* buffer.

The attached patch fixes it, but there are other possible ways to do it;
pass the project to xref--analyze, or set a local variable containing
the project.

In addition, refresh-buffer in the *xref* buffer will have the same
problem, so setting the original project in a local variable might be
the best way.

-- 
-- Stephe