Merge branch 'jh/filter-empty-contents' into maint

The clean/smudge interface did not work well when filtering an empty contents (failed and then passed the empty input through). It can be argued that a filter that produces anything but empty for an empty input is nonsense, but if the user wants to do strange things, then why not? * jh/filter-empty-contents: sha1_file: pass empty buffer to index empty file