specifying ranges: we did not mean to make ".." an empty set

Either end of revision range operator can be omitted to default to HEAD,
as in "origin.." (what did I do since I forked) or "..origin" (what did
they do since I forked).  But the current parser interprets ".."  as an
empty range "HEAD..HEAD", and worse yet, because ".." does exist on the
filesystem, we get this annoying output:

  $ cd Documentation/howto
  $ git log .. ;# give me recent commits that touch Documentation/ area.
  fatal: ambiguous argument '..': both revision and filename
  Use '--' to separate filenames from revisions

Surely we could say "git log ../" or even "git log -- .." to disambiguate,
but we shouldn't have to.

Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 14 insertions, 2 deletions

diff --git a/revision.c b/revision.c
index 5b81a92..457868d 100644
--- a/revision.c
+++ b/revision.c
@@ -1132,15 +1132,27 @@ 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";
 		unsigned int a_flags;
 
 		*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;