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

@@ -41,6 +41,9 b' filediff(){'
     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
@@ -53,6 +56,14 b' if test -d "$1" -o -d "$2"; then'
     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