merge-ort: write $GIT_DIR/AUTO_MERGE whenever we hit a conflict

There are a variety of questions users might ask while resolving
conflicts:
  * What changes have been made since the previous (first) parent?
  * What changes are staged?
  * What is still unstaged? (or what is still conflicted?)
  * What changes did I make to resolve conflicts so far?
The first three of these have simple answers:
  * git diff HEAD
  * git diff --cached
  * git diff
There was no way to answer the final question previously.  Adding one
is trivial in merge-ort, since it works by creating a tree representing
what should be written to the working copy complete with conflict
markers.  Simply write that tree to .git/AUTO_MERGE, allowing users to
answer the fourth question with
  * git diff AUTO_MERGE

I avoided using a name like "MERGE_AUTO", because that would be
merge-specific (much like MERGE_HEAD, REBASE_HEAD, REVERT_HEAD,
CHERRY_PICK_HEAD) and I wanted a name that didn't change depending on
which type of operation the merge was part of.

Ensure that paths which clean out other temporary operation-specific
files (e.g. CHERRY_PICK_HEAD, MERGE_MSG, rebase-merge/ state directory)
also clean out this AUTO_MERGE file.

Signed-off-by: Elijah Newren <newren@gmail.com>
Reviewed-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 2 insertions, 0 deletions

diff --git a/path.h b/path.h
index e7e77da..251c78d 100644
--- a/path.h
+++ b/path.h
@@ -176,6 +176,7 @@ struct path_cache {
 	const char *merge_mode;
 	const char *merge_head;
 	const char *merge_autostash;
+	const char *auto_merge;
 	const char *fetch_head;
 	const char *shallow;
 };
@@ -191,6 +192,7 @@ const char *git_path_merge_rr(struct repository *r);
 const char *git_path_merge_mode(struct repository *r);
 const char *git_path_merge_head(struct repository *r);
 const char *git_path_merge_autostash(struct repository *r);
+const char *git_path_auto_merge(struct repository *r);
 const char *git_path_fetch_head(struct repository *r);
 const char *git_path_shallow(struct repository *r);