Merge branch 'jc/add-resolved'

Merge branch 'jc/add-resolved'

'git add' has been taught a new '--resolved' option to stage
conflict-resolved paths, while leaving unrelated local changes
unstaged.  It scans the unmerged paths for leftover conflict
markers and aborts if any are found.

* jc/add-resolved:
  add: introduce '--resolved' option
  read-cache: add remove_file_from_index_with_flags()
  merge-ll: consolidate conflict marker scanning logic
  read-cache: reindent