Fix diff-mode face problem when used in terminals (Bug#35695)

In a terminal supporting 256 colors, both diff-added and diff-removed
was mapped to the same greyish color.

* lisp/vc/diff-mode.el: Modify the colors of diff-removed,
  diff-added, diff-refine-removed, and diff-refine-added when
  used in a 256 color environment.
1 job for master in 51 minutes and 29 seconds (queued for 3 seconds)
Status Job ID Name Coverage
failed #1771


Name Stage Failure
test-all Test
Makefile:318: recipe for target 'check-doit' failed
make[2]: *** [check-doit] Error 1
make[2]: Leaving directory '/builds/emacs/emacs/test'
Makefile:293: recipe for target 'check-expensive' failed
make[1]: *** [check-expensive] Error 2
make[1]: Leaving directory '/builds/emacs/emacs/test'
Makefile:957: recipe for target 'check-expensive' failed
make: *** [check-expensive] Error 2
ERROR: Job failed: exit code 1