(vc-do-command): If the destination buffer is already current, don't

(vc-do-command): If the destination buffer is already current, don't
set it up and don't erase it.  Updated doc string to reflect that.
(vc-version-diff): When doing a single file diff, don't switch to the
*vc-diff* buffer before the command.  This is to preserve local variable
bindings that might affect the diff command.
(vc-print-log): Don't switch to the *vc* buffer before the command, so
that local variable bindings are preserved.