Merge branch 'zh/ls-files-deduplicate'

"git ls-files" can and does show multiple entries when the index is
unmerged, which is a source for confusion unless -s/-u option is in
use.  A new option --deduplicate has been introduced.

* zh/ls-files-deduplicate:
  ls-files.c: add --deduplicate option
  ls_files.c: consolidate two for loops into one
  ls_files.c: bugfix for --deleted and --modified