revision arguments: ..B means HEAD..B, just like A.. means A..HEAD

For consistency reasons, we should probably allow that to be written as
just "..branch", the same way we can write "branch.." to mean "everything
in HEAD but not in "branch".
Signed-off-by: NJunio C Hamano <junkio@cox.net>

rev-parse.c

@@ -315,16 +315,17 @@ int main(int argc, char **argv)
 		dotdot = strstr(arg, "..");
 		if (dotdot) {
 			unsigned char end[20];
-			char *n = dotdot+2;
+			char *next = dotdot + 2;
+			char *this = arg;
 			*dotdot = 0;
-			if (!get_sha1(arg, sha1)) {
-				if (!*n)
-					n = "HEAD";
-				if (!get_sha1(n, end)) {
-					show_rev(NORMAL, end, n);
-					show_rev(REVERSED, sha1, arg);
-					continue;
-				}
+			if (!*next)
+				next = "HEAD";
+			if (dotdot == arg)
+				this = "HEAD";
+			if (!get_sha1(this, sha1) && !get_sha1(next, end)) {
+				show_rev(NORMAL, end, next);
+				show_rev(REVERSED, sha1, this);
+				continue;
 			}
 			*dotdot = '.';
 		}

revision.c

@@ -642,14 +642,19 @@ int setup_revisions(int argc, const char **argv, struct rev_info *revs, const ch
 		if (dotdot) {
 			unsigned char from_sha1[20];
 			char *next = dotdot + 2;
+			char *this = arg;
+			static const char HEAD[] = "HEAD";
 			*dotdot = 0;
 			if (!*next)
-				next = "HEAD";
-			if (!get_sha1(arg, from_sha1) && !get_sha1(next, sha1)) {
+				next = HEAD;
+			if (dotdot == arg)
+				this = HEAD;
+			if (!get_sha1(this, from_sha1) &&
+			    !get_sha1(next, sha1)) {
 				struct commit *exclude;
 				struct commit *include;
 
-				exclude = get_commit_reference(revs, arg, from_sha1, flags ^ UNINTERESTING);
+				exclude = get_commit_reference(revs, this, from_sha1, flags ^ UNINTERESTING);
 				include = get_commit_reference(revs, next, sha1, flags);
 				if (!exclude || !include)
 					die("Invalid revision range %s..%s", arg, next);