VC includes a function vc-do-command which invokes a version control
command.  This function uses call-process and therefore only works for
local files.

Tramp includes tramp-vc-do-command, a version of vc-do-command which
invokes shell-command instead of call-process and therefore works for
remote files, too.  Tramp then advises vc-do-command to invoke
tramp-vc-do-command on Tramp files.

Some years ago, we discussed replacing this kludge with an enhanced
call-process that also groks file handlers.  Richard didn't want to
change the behavior of call-process and proposed to provide a new
function process-file instead.

The attached patch provides the new function process-file, adds Tramp
support for this new function, and changes VC to invoke process-file
instead of call-process.  The patch also removes the ugly advice from
Tramp if process-file is available.

Beware: the Tramp implementation for process-file is far from
complete.  It appears to be sufficient for vc-do-command.  I intend to
improve the implementation somewhat if the general approach is
agreeable.

What do people think?

Kai