Improve prefix arg support in 'ediff-scroll-horizontally'

* lisp/vc/ediff-util.el (ediff-scroll-horizontally): Use
'current-prefix-arg' to pass the value of prefix argument to
scrolling commands.  (Bug#39353)

Copyright-paperwork-exempt: yes
1 job for master in 72 minutes and 12 seconds (queued for 4 seconds)
Status Job ID Name Coverage
  Test
passed #4977
test-all

01:12:12