bug#25992: perform-replace leaves mark-active when not transient-mark-mode

[Keith David Bershatsky]

When transient-mark-mode is turned off, perform-replace leaves mark-active set 
to `t`.

Step 1:  Launch emacs -q

Step 2:  Turn off transient-mark-mode.  (transient-mark-mode -1)

Step 3:  Evaluate mark-active and verify it is `nil`.

Step 4:  Run a simple perform-replace such as:  (replace-regexp ";" "@" nil 1 2)

Step 5:  Evaluate mark-active and see that it is now set `t`.

The desired behavior is to leave `mark-active` set to `nil` if it was 
previously set to `nil`.

I fixed this in my own setup with a very simple test at the outset of 
perform-replace to see whether mark-active was set to `t` at the outset and 
then turn it off at the end of the function if it was not previously active.

Thanks,

Keith