[PATCH v2] specifying ranges: we did not mean to make ".." an empty set

@@ -184,6 +184,13 @@ of `r1` and `r2` and is defined as
 It is the set of commits that are reachable from either one of
 `r1` or `r2` but not from both.
 
+In these two shorthands, you can omit one end and let it default to HEAD.
+For example, 'origin..' is a shorthand for 'origin..HEAD' and asks "What
+did I do since I forked from the origin branch?"  Similarly, '..origin'
+is a shorthand for 'HEAD..origin' and asks "What did the origin do since
+I forked from them?"  Note that you cannot omit both ends.  '..' is not
+an empty range that is both reachable and unreachable from HEAD.
+
 Two other shorthands for naming a set that is formed by a commit
 and its parent commits exist.  The `r1{caret}@` notation means all
 parents of `r1`.  `r1{caret}!` includes commit `r1` but excludes

@@ -219,6 +219,7 @@ static int try_difference(const char *arg)
 	const char *next;
 	const char *this;
 	int symmetric;
+	static const char head_by_default[] = "HEAD";
 
 	if (!(dotdot = strstr(arg, "..")))
 		return 0;

@@ -230,9 +231,20 @@ static int try_difference(const char *arg)
 	next += symmetric;
 
 	if (!*next)
-		next = "HEAD";
+		next = head_by_default;
 	if (dotdot == arg)
-		this = "HEAD";
+		this = head_by_default;
+
+	if (this == head_by_default && next == head_by_default &&
+	    !symmetric) {
+		/*
+		 * Just ".."?  That is not a range but the
+		 * pathspec for the parent directory.
+		 */
+		*dotdot = '.';
+		return 0;
+	}
+
 	if (!get_sha1(this, sha1) && !get_sha1(next, end)) {
 		show_rev(NORMAL, end, next);
 		show_rev(symmetric ? NORMAL : REVERSED, sha1, this);

@@ -1004,14 +1004,26 @@ int handle_revision_arg(const char *arg, struct rev_info *revs,
 		const char *this = arg;
 		int symmetric = *next == '.';
 		unsigned int flags_exclude = flags ^ UNINTERESTING;
+		static const char head_by_default[] = "HEAD";
 
 		*dotdot = 0;
 		next += symmetric;
 
 		if (!*next)
-			next = "HEAD";
+			next = head_by_default;
 		if (dotdot == arg)
-			this = "HEAD";
+			this = head_by_default;
+		if (this == head_by_default && next == head_by_default &&
+		    !symmetric) {
+			/*
+			 * Just ".."?  That is not a range but the
+			 * pathspec for the parent directory.
+			 */
+			if (!cant_be_filename) {
+				*dotdot = '.';
+				return -1;
+			}
+		}
 		if (!get_sha1(this, from_sha1) &&
 		    !get_sha1(next, sha1)) {
 			struct commit *a, *b;

@@ -75,4 +75,18 @@ test_expect_success 'invalid @{n} reference' '
 	grep "fatal: Log for [^ ]* only has [0-9][0-9]* entries." error
 '
 
+test_expect_success 'dotdot is not an empty set' '
+	( H=$(git rev-parse HEAD) && echo $H ; echo ^$H ) >expect &&
+
+	git rev-parse HEAD.. >actual &&
+	test_cmp expect actual &&
+
+	git rev-parse ..HEAD >actual &&
+	test_cmp expect actual &&
+
+	echo .. >expect &&
+	git rev-parse .. >actual &&
+	test_cmp expect actual
+'
+
 test_done

@@ -463,4 +463,11 @@ test_expect_success 'show added path under "--follow -M"' '
 	)
 '
 
+test_expect_success 'dotdot is a parent directory' '
+	mkdir -p a/b &&
+	( echo sixth && echo fifth ) >expect &&
+	( cd a/b && git log --format=%s .. ) >actual &&
+	test_cmp expect actual
+'
+
 test_done