[Sass] Make sass-convert -F sass -T sass require --in-place

Closes sassgh-137

doc-src/SASS_CHANGELOG.md

@@ -13,6 +13,12 @@ now allow whitespace before the colon. For example:
     foo
       color : blue
 
+### Minor Changes
+
+* Running `sass-convert --from sass --to sass` will raise an error
+  unless `--in-place` is provided,
+  to prevent accidental erasure of source files.
+
 ### Bug Fixes
 
 * Variables are now allowed as arguments to `url()`.

lib/haml/exec.rb

@@ -666,6 +666,11 @@ def process_directory
         end
         @options[:output] ||= @options[:input]
 
+        if @options[:to] == @options[:from] && !@options[:in_place]
+          fmt = @options[:from]
+          raise "Error: converting from #{fmt} to #{fmt} without --in-place"
+        end
+
         ext = @options[:from]
         ext = :sass if ext == :sass2
         Dir.glob("#{@options[:input]}/**/*.#{ext}") do |f|