[PATCH v3 0/2] merge-tree: add new --dry-run option

Changes since v2:

 * Converted locations missed in v1 in changing --mergeability-only ->
   --dry-run

Changes since v1:

 * Renamed --mergeability-only flag to --dry-run, as per suggestion from
   Junio
 * added some commit message clarifications

This adds a new flag, --dry-run, to git merge-tree, which suppresses all output and leaves only the exit status (reflecting successful merge or conflict). This is useful for Git Forges in cases where they are only interested in whether two branches can be merged, without needing the actual merge result or conflict details.

The advantage of the flag is two fold:

 * The merge machinery can exit once it detects a conflict, instead of
   continuing to compute merge result information
 * The merge machinery can avoid writing merged blobs and trees to the
   object store when in the outer layer of the merging process (more details
   in the first commit message).

Elijah Newren (2):
  merge-ort: add a new mergeability_only option
  merge-tree: add a new --dry-run flag

 Documentation/git-merge-tree.adoc |  6 +++++
 builtin/merge-tree.c              | 22 ++++++++++++++++++
 merge-ort.c                       | 38 +++++++++++++++++++++++++------
 merge-ort.h                       |  1 +
 t/t4301-merge-tree-write-tree.sh  | 38 +++++++++++++++++++++++++++++++
 5 files changed, 98 insertions(+), 7 deletions(-)

base-commit: 6c0bd1fc70efaf053abe4e57c976afdc72d15377
Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-1920%2Fnewren%2Fmergeability-only-v3
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-1920/newren/mergeability-only-v3
Pull-Request: https://github.com/gitgitgadget/git/pull/1920

Range-diff vs v2:

 1:  4757c4810d3 = 1:  4757c4810d3 merge-ort: add a new mergeability_only option
 2:  1d18ab7feb8 ! 2:  f11824317a8 merge-tree: add a new --dry-run flag
     @@ Documentation/git-merge-tree.adoc: OPTIONS
       	default is to include these messages if there are merge
       	conflicts, and to omit them otherwise.
       
     -+--mergeability-only::
     ++--dry-run::
      +	Disable all output from the program.  Useful when you are only
      +	interested in the exit status.  Allows merge-tree to exit
     -+	early on the first conflict it finds, and allows it to avoid
     -+	writing most objects created by merges.
     ++	early when it finds a conflict, and allows it to avoid writing
     ++	most objects created by merges.
      +
       --allow-unrelated-histories::
       	merge-tree will by default error out if the two branches specified
     @@ builtin/merge-tree.c: int cmd_merge_tree(int argc,
      +	if (dry_run && o.show_messages == -1)
      +		o.show_messages = 0;
      +	o.merge_options.mergeability_only = dry_run;
     -+	die_for_incompatible_opt2(dry_run, "--mergeability-only",
     ++	die_for_incompatible_opt2(dry_run, "--dry-run",
      +				  o.show_messages, "--messages");
     -+	die_for_incompatible_opt2(dry_run, "--mergeability-only",
     ++	die_for_incompatible_opt2(dry_run, "--dry-run",
      +				  o.name_only, "--name-only");
     -+	die_for_incompatible_opt2(dry_run, "--mergeability-only",
     ++	die_for_incompatible_opt2(dry_run, "--dry-run",
      +				  o.use_stdin, "--stdin");
     -+	die_for_incompatible_opt2(dry_run, "--mergeability-only",
     ++	die_for_incompatible_opt2(dry_run, "--dry-run",
      +				  !line_termination, "-z");
      +
       	if (xopts.nr && o.mode == MODE_TRIVIAL)