Work properly with '-' or /dev/stdin

/dev/stdin will be treated as normal file, albeit `test -s` would detect
it as non-existent.  '-' will cause STDIN to be used as wellm but will
omit header.

src/include/pretty.sh

@@ -335,9 +335,14 @@ _pretty__echo_files() {
     local fp
     for fp in "$@";
     do
-        test -s "$fp" || continue
-        echo "-- $fp --"
-        cat "$fp"
+        if test "$fp" = "-";
+        then
+            cat
+        elif test -s "$fp" || test "$fp" = "/dev/stdin";
+        then
+            echo "-- $fp --"
+            cat "$fp"
+        fi
     done
 }