I think Linus did a cut & paste from an early JIT code while
developing the current extended SHA1 notation, and left it there as a
courtesy, but the directory does not deserve to be treated any more
specially than, say, .git/refs/bisect.

If the subdirectories under .git/refs proliferate, we may want to
switch to scanning that hierarchy at runtime, instead of the current
hard-coded set, although I think that would be overkill.
Signed-off-by: NJunio C Hamano <junkio@cox.net>
From nobody Mon Sep 17 00:00:00 2001
Subject: [PATCH] Add a new extended SHA1 syntax <name>:<num>
From: Junio C Hamano <junkio@cox.net>
Date: 1124617434 -0700

The new notation is a short-hand for <name> followed by <num>
caret ('^') characters.  E.g. "master:4" is the fourth
generation ancestor of the current "master" branch head,
following the first parents; same as "master^^^^" but a bit more
readable.

This will be used in the updated "git show-branch" command.
Signed-off-by: NJunio C Hamano <junkio@cox.net>

---

 sha1_name.c |   41 +++++++++++++++++++++++++++++++++++++++++
 1 files changed, 41 insertions(+), 0 deletions(-)

d5098ce769da46df6d45dc8f41b06dd758fdaea7
diff --git a/sha1_name.c b/sha1_name.c
--- a/sha1_name.c
+++ b/sha1_name.c
@@ -191,9 +191,29 @@ static int get_parent(const char *name, 
 	return -1;
 }
 
+static int get_nth_ancestor(const char *name, int len,
+			    unsigned char *result, int generation)
+{
+	unsigned char sha1[20];
+	int ret = get_sha1_1(name, len, sha1);
+	if (ret)
+		return ret;
+
+	while (generation--) {
+		struct commit *commit = lookup_commit_reference(sha1);
+
+		if (!commit || parse_commit(commit) || !commit->parents)
+			return -1;
+		memcpy(sha1, commit->parents->item->object.sha1, 20);
+	}
+	memcpy(result, sha1, 20);
+	return 0;
+}
+
 static int get_sha1_1(const char *name, int len, unsigned char *sha1)
 {
 	int parent, ret;
+	const char *cp;
 
 	/* foo^[0-9] or foo^ (== foo^1); we do not do more than 9 parents. */
 	if (len > 2 && name[len-2] == '^' &&
@@ -210,6 +230,27 @@ static int get_sha1_1(const char *name, 
 	if (parent >= 0)
 		return get_parent(name, len, sha1, parent);
 
+	/* name:3 is name^^^,
+	 * name:12 is name^^^^^^^^^^^^, and
+	 * name: is name
+	 */
+	parent = 0;
+	for (cp = name + len - 1; name <= cp; cp--) {
+		int ch = *cp;
+		if ('0' <= ch && ch <= '9')
+			continue;
+		if (ch != ':')
+			parent = -1;
+		break;
+	}
+	if (!parent && *cp == ':') {
+		int len1 = cp - name;
+		cp++;
+		while (cp < name + len)
+			parent = parent * 10 + *cp++ - '0';
+		return get_nth_ancestor(name, len1, sha1, parent);
+	}
+
 	ret = get_sha1_basic(name, len, sha1);
 	if (!ret)
 		return 0;