tests: make pdiff return appropriate exit code at comparison of files

Before this patch, pdiff script returns 0, even if diff is detected.

This issue doesn't cause failure of tests using it, if it is invoked
via extdiff extension, because extdiff itself examines changes between
specified revisions and decides exit code.

BTW, this patch ignores recursive comparison case, because:

  - there is no portable way for current while-read based
    implementation to return 1 at detecting changes

  - it isn't yet needed to replace direct "diff -r" invocation by
    pdiff for portability

tests/pdiff

@@ -1,58 +1,69 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" 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@"
+
+    # in this case, files differ from each other
+    return 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
+
+    # TODO: there is no portable way for current while-read based
+    # implementation to return 1 at detecting changes.
+    #
+    # On bash and dash, assignment to variable inside while-block
+    # doesn't affect outside, because inside while-block is executed
+    # in sub-shell. BTW, it affects outside while-block on ksh (as sh
+    # on Solaris).
 else
     filediff "$1" "$2"
 fi