bug#5296: vc-rcs rcsdiff -u1 -> -u -1

[Dan Nicolaescu]

karl@freefriends.org (Karl Berry) writes:

  > In Emacs 23.1, doing vc-diff on an RCS file results in an error with
  > GNU diff 2.8.1 (or later):
  > 
  > /usr/bin/diff: `-1' option is obsolete; use `-U 1'                          
  > ..
  > 
  > The *Messages* buffer reports:
  > vc-do-command: Running rcsdiff -q   -p -u1 maintainers...FAILED (status 2)
  > 
  > I could not quickly tell where the invocation was in vc-rcs.el, but
  > afaik replacing -u1 with -u 1 (or -U 1) is the solution.  (The workaround 
for
  > users is to set _POSIX2_VERSION=0 in the environment.)

Do you set `diff-switches' to "-u1" ?

This call:
(vc-switches 'RCS 'diff)
in `vc-rcs-diff' uses `diff-switches', and passes the result to "rcsdiff".

Not sure what to do here.  We already have a bug:
#4422: diff-switches is a bad idea