Merge branch 'jc/blame-reverse' into next

It is a common mistake to say "git blame --reverse OLD path",
expecting that the command line is dwimmed as if asking how lines
in path in an old revision OLD have survived up to the current
commit.

Has been waiting for positive responses without seeing any.

* jc/blame-reverse:
  blame: dwim "blame --reverse OLD" as "blame --reverse OLD.."
  blame: improve diagnosis for "--reverse NEW"

Author: gitster

Documentation/blame-options.txt

@@ -28,12 +28,13 @@ include::line-range-format.txt[]
 -S <revs-file>::
 	Use revisions from revs-file instead of calling linkgit:git-rev-list[1].
 
---reverse::
+--reverse <rev>..<rev>::
 	Walk history forward instead of backward. Instead of showing
 	the revision in which a line appeared, this shows the last
 	revision in which a line has existed. This requires a range of
 	revision like START..END where the path to blame exists in
-	START.
+	START.  `git blame --reverse START` is taken as `git blame
+	--reverse START..HEAD` for convenience.
 
 -p::
 --porcelain::

Documentation/git-blame.txt

@@ -10,7 +10,7 @@ SYNOPSIS
 [verse]
 'git blame' [-c] [-b] [-l] [--root] [-t] [-f] [-n] [-s] [-e] [-p] [-w] [--incremental]
 	    [-L <range>] [-S <revs-file>] [-M] [-C] [-C] [-C] [--since=<date>]
-	    [--progress] [--abbrev=<n>] [<rev> | --contents <file> | --reverse <rev>]
+	    [--progress] [--abbrev=<n>] [<rev> | --contents <file> | --reverse <rev>..<rev>]
 	    [--] <file>
 
 DESCRIPTION

builtin/blame.c

@@ -2456,6 +2456,41 @@ static char *prepare_final(struct scoreboard *sb)
 	return xstrdup_or_null(name);
 }
 
+static const char *dwim_reverse_initial(struct scoreboard *sb)
+{
+	/*
+	 * DWIM "git blame --reverse ONE -- PATH" as
+	 * "git blame --reverse ONE..HEAD -- PATH" but only do so
+	 * when it makes sense.
+	 */
+	struct object *obj;
+	struct commit *head_commit;
+	unsigned char head_sha1[20];
+
+	if (sb->revs->pending.nr != 1)
+		return NULL;
+
+	/* Is that sole rev a committish? */
+	obj = sb->revs->pending.objects[0].item;
+	obj = deref_tag(obj, NULL, 0);
+	if (obj->type != OBJ_COMMIT)
+		return NULL;
+
+	/* Do we have HEAD? */
+	if (!resolve_ref_unsafe("HEAD", RESOLVE_REF_READING, head_sha1, NULL))
+		return NULL;
+	head_commit = lookup_commit_reference_gently(head_sha1, 1);
+	if (!head_commit)
+		return NULL;
+
+	/* Turn "ONE" into "ONE..HEAD" then */
+	obj->flags |= UNINTERESTING;
+	add_pending_object(sb->revs, &head_commit->object, "HEAD");
+
+	sb->final = (struct commit *)obj;
+	return sb->revs->pending.objects[0].name;
+}
+
 static char *prepare_initial(struct scoreboard *sb)
 {
 	int i;
@@ -2474,14 +2509,17 @@ static char *prepare_initial(struct scoreboard *sb)
 		if (obj->type != OBJ_COMMIT)
 			die("Non commit %s?", revs->pending.objects[i].name);
 		if (sb->final)
-			die("More than one commit to dig down to %s and %s?",
+			die("More than one commit to dig up from, %s and %s?",
 			    revs->pending.objects[i].name,
 			    final_commit_name);
 		sb->final = (struct commit *) obj;
 		final_commit_name = revs->pending.objects[i].name;
 	}
+
+	if (!final_commit_name)
+		final_commit_name = dwim_reverse_initial(sb);
 	if (!final_commit_name)
-		die("No commit to dig down to?");
+		die("No commit to dig up from?");
 	return xstrdup(final_commit_name);
 }