[PATCH v2] revision: add separate field for "-m" of "diff-index -m"

[Sergey Organov <sorganov@gmail.com>]

Historically, in "diff-index -m", "-m" does not mean "do not ignore merges", but
"match missing". Despite this, diff-index abuses 'ignore_merges' field being set
by "-m", that in turn causes more troubles.

Add separate 'diff_index_match_missing' field for diff-index to use and set it
when we encounter "-m" option. This field won't then be cleared when primary
meaning of "-m" is reverted (e.g., by "--no-diff-merges"), nor it will be
affected by future option(s) that might drive 'ignore_merges' field.

Use this new field from diff-lib:do_oneway_diff() instead of abusing
'ignore_merges' field.

Signed-off-by: Sergey Organov <sorganov@gmail.com>
---

v2: rebased from 'maint' onto 'master'

 diff-lib.c | 10 ++--------
 revision.c |  6 ++++++
 revision.h |  1 +
 3 files changed, 9 insertions(+), 8 deletions(-)

diff --git a/diff-lib.c b/diff-lib.c
index 50521e2093fc..f2aee78e7aa2 100644
--- a/diff-lib.c
+++ b/diff-lib.c
@@ -405,14 +405,8 @@ static void do_oneway_diff(struct unpack_trees_options *o,
 	/* if the entry is not checked out, don't examine work tree */
 	cached = o->index_only ||
 		(idx && ((idx->ce_flags & CE_VALID) || ce_skip_worktree(idx)));
-	/*
-	 * Backward compatibility wart - "diff-index -m" does
-	 * not mean "do not ignore merges", but "match_missing".
-	 *
-	 * But with the revision flag parsing, that's found in
-	 * "!revs->ignore_merges".
-	 */
-	match_missing = !revs->ignore_merges;
+
+	match_missing = revs->diff_index_match_missing;
 
 	if (cached && idx && ce_stage(idx)) {
 		struct diff_filepair *pair;
diff --git a/revision.c b/revision.c
index 96630e31867d..64b16f7d1033 100644
--- a/revision.c
+++ b/revision.c
@@ -2345,6 +2345,12 @@ static int handle_revision_opt(struct rev_info *revs, int argc, const char **arg
 		revs->diffopt.flags.tree_in_recursive = 1;
 	} else if (!strcmp(arg, "-m")) {
 		revs->ignore_merges = 0;
+		/*
+		 * Backward compatibility wart - "diff-index -m" does
+		 * not mean "do not ignore merges", but "match_missing",
+		 * so set separate flag for it.
+		 */
+		revs->diff_index_match_missing = 1;
 	} else if ((argcount = parse_long_opt("diff-merges", argv, &optarg))) {
 		if (!strcmp(optarg, "off")) {
 			revs->ignore_merges = 1;
diff --git a/revision.h b/revision.h
index c1e5bcf139d7..5ae8254ffaed 100644
--- a/revision.h
+++ b/revision.h
@@ -188,6 +188,7 @@ struct rev_info {
 	unsigned int	diff:1,
 			full_diff:1,
 			show_root_diff:1,
+			diff_index_match_missing:1,
 			no_commit_id:1,
 			verbose_header:1,
 			combine_merges:1,
-- 
2.25.1