tests: Solaris cmp complains about empty files, even with -s

When you compare an empty file, such as /dev/null, with a non-empty file,
Solaris cmp complains on stderr with "cmp: EOF on /dev/null", even if the
-s argument is present.  GNU cmp makes the complaint, but silences it with
-s.  We can change the pdiff utility to simply redirect stderr to /dev/null
so that we don't have to worry about this difference in the test files.

tests/pdiff

@@ -1,58 +1,58 b''
 #!/bin/sh
 
 # Script to get stable diff output on any platform.
 #
 # Output of this script is almost equivalent to GNU diff with "-Nru".
 #
 # Use this script as "hg pdiff" via extdiff extension with preparation
 # below in test scripts:
 #
 #   $ cat >> $HGRCPATH <<EOF
 #   > [extdiff]
 #   > pdiff = sh "$RUNTESTDIR/pdiff"
 #   > EOF
 
 filediff(){
     # USAGE: filediff file1 file2 [header]
 
     # compare with /dev/null if file doesn't exist (as "-N" option)
     file1="$1"
     if test ! -f "$file1"; then
         file1=/dev/null
     fi
     file2="$2"
     if test ! -f "$file2"; then
         file2=/dev/null
     fi
 
-    if cmp -s "$file1" "$file2"; then
+    if cmp -s "$file1" "$file2" 2> /dev/null; then
         # Return immediately, because comparison isn't needed. This
         # also avoids redundant message of diff like "No differences
         # encountered" (on Solaris)
         return
     fi
 
     if test -n "$3"; then
         # show header only in recursive case
         echo "$3"
     fi
 
     # replace "/dev/null" by corresponded filename (as "-N" option)
     diff -u "$file1" "$file2" |
     sed "s@^--- /dev/null\(.*\)\$@--- $1\1@" |
     sed "s@^\+\+\+ /dev/null\(.*\)\$@+++ $2\1@"
 }
 
 if test -d "$1" -o -d "$2"; then
     # ensure comparison in dictionary order
     (
     if test -d "$1"; then (cd "$1" && find . -type f); fi
     if test -d "$2"; then (cd "$2" && find . -type f); fi
     ) |
     sed 's@^\./@@g' | sort | uniq |
     while read file; do
         filediff "$1/$file" "$2/$file" "diff -Nru $1/$file $2/$file"
     done
 else
     filediff "$1" "$2"
 fi