[PATCH 0/8] RFC: Server side merges (no ref updating, no commit creating, no touching worktree or index)

(NOTE for Junio: This series has a minor conflict with en/remerge-diff -- this series moves a code block into a new function, but en/remerge-diff adds a BUG() message to that code block. But this series is just RFC, so you may want to wait to pick it up.)

NOTE2: A preliminary version of this series was discussed here:
https://lore.kernel.org/git/nycvar.QRO.7.76.6.2110211147490.56@tvgsbejvaqbjf.bet/

This series introduces a new option to git-merge-tree: --real (best name I could come up with). This new option is designed to allow a server-side "real" merge (or allow folks client-side to do merges with branches they don't even have checked out). Real merges differ from trivial merges in that they handle:

 * three way content merges
 * recursive ancestor consolidation
 * renames
 * proper directory/file conflict handling
 * etc.

The reason this is different from merge is that merge-tree does NOT:

 * Read/write/update any working tree (and assumes there probably isn't one)
 * Read/write/update any index (and assumes there probably isn't one)
 * Create a commit object
 * Update any refs

This series attempts to guess what kind of output would be wanted, basically choosing:

 * clean merge or conflict signalled via exit status
 * stdout consists solely of printing the hash of the resulting tree (though
   that tree may include files that have conflict markers)
 * new optional --messages flag for specifying a file where informational
   messages (e.g. conflict notices and files involved in three-way-content
   merges) can be written; by default, this output is simply discarded
 * new optional --conflicted-list flag for specifying a file where the names
   of conflicted-files can be written in a NUL-character-separated list

This design means it's basically just a low-level tool that other scripts would use and do additional work with. Perhaps something like this:

   NEWTREE=$(git merge-tree --real $BRANCH1 $BRANCH2)
   test $? -eq 0 || die "There were conflicts..."
   NEWCOMMIT=$(git commit-tree $NEWTREE -p $BRANCH1 $BRANCH2)
   git update-ref $BRANCH1 $NEWCOMMIT

Elijah Newren (8):
  merge-tree: rename merge_trees() to trivial_merge_trees()
  merge-tree: move logic for existing merge into new function
  merge-tree: add option parsing and initial shell for real merge
    function
  merge-tree: implement real merges
  merge-ort: split out a separate display_update_messages() function
  merge-ort: allow update messages to be written to different file
    stream
  merge-tree: support saving merge messages to a separate file
  merge-tree: provide an easy way to access which files have conflicts

 Documentation/git-merge-tree.txt |  32 +++++--
 builtin/merge-tree.c             | 152 ++++++++++++++++++++++++++++---
 git.c                            |   2 +-
 merge-ort.c                      |  85 ++++++++++-------
 merge-ort.h                      |  12 +++
 t/t4301-merge-tree-real.sh       | 108 ++++++++++++++++++++++
 6 files changed, 333 insertions(+), 58 deletions(-)
 create mode 100755 t/t4301-merge-tree-real.sh

base-commit: 2ae0a9cb8298185a94e5998086f380a355dd8907
Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-git-1114%2Fnewren%2Fmerge-into-v1
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-git-1114/newren/merge-into-v1
Pull-Request: https://github.com/git/git/pull/1114