Merge branch 'en/merge-tree'

Merge branch 'en/merge-tree'

"git merge-tree" learned a new mode where it takes two commits and
computes a tree that would result in the merge commit, if the
histories leading to these two commits were to be merged.

* en/merge-tree:
  git-merge-tree.txt: add a section on potentional usage mistakes
  merge-tree: add a --allow-unrelated-histories flag
  merge-tree: allow `ls-files -u` style info to be NUL terminated
  merge-ort: optionally produce machine-readable output
  merge-ort: store more specific conflict information
  merge-ort: make `path_messages` a strmap to a string_list
  merge-ort: store messages in a list, not in a single strbuf
  merge-tree: provide easy access to `ls-files -u` style info
  merge-tree: provide a list of which files have conflicts
  merge-ort: remove command-line-centric submodule message from merge-ort
  merge-ort: provide a merge_get_conflicted_files() helper function
  merge-tree: support including merge messages in output
  merge-ort: split out a separate display_update_messages() function
  merge-tree: implement real merges
  merge-tree: add option parsing and initial shell for real merge function
  merge-tree: move logic for existing merge into new function
  merge-tree: rename merge_trees() to trivial_merge_trees()